Source code editor What Is Ajax
Source code editor What Is Ajax
↑
Table of Contents
This chapter describes the syntax for most of the SQL statements supported by MySQL. Additional statement descriptions can be found in the following chapters:
The EXPLAIN statement is discussed in Chapter 7, Optimization.
Statements for writing stored routines are covered in Chapter 17, Stored Procedures and Functions.
Statements for writing triggers are covered in Chapter 18, Triggers.
View-related statements are covered in Chapter 19, Views.
ALTER {DATABASE | SCHEMA} [db_name]
alter_specification [alter_specification] ...
alter_specification:
[DEFAULT] CHARACTER SET charset_name
| [DEFAULT] COLLATE collation_name ALTER DATABASE enables you to change the overall characteristics of a database. These characteristics are stored in the db.opt file in the database directory. To use ALTER DATABASE, you need the ALTER privilege on the database. ALTER SCHEMA is a synonym for ALTER DATABASE as of MySQL 5.0.2.
The CHARACTER SET clause changes the default database character set. The COLLATE clause changes the default database collation. Chapter 10, Character Set Support, discusses character set and collation names.
The database name can be omitted, in which case the statement applies to the default database.
MySQL Enterprise. In a production environment, alteration of a database is not a common occurrence and may indicate a security breach. Advisors provided as part of the MySQL Network Monitoring and Advisory Service automatically alert you when data definition statements are issued. For more information see, http://www.mysql.com/products/enterprise/advisors.html.
ALTER [IGNORE] TABLEtbl_namealter_specification[,alter_specification] ...alter_specification:table_option... | ADD [COLUMN]column_definition[FIRST | AFTERcol_name] | ADD [COLUMN] (column_definition,...) | ADD {INDEX|KEY} [index_name] [index_type] (index_col_name,...) | ADD [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...) | ADD [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...) | ADD [FULLTEXT|SPATIAL] [INDEX|KEY] [index_name] (index_col_name,...) | ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name,...) [reference_definition] | ALTER [COLUMN]col_name{SET DEFAULTliteral| DROP DEFAULT} | CHANGE [COLUMN]old_col_namecolumn_definition[FIRST|AFTERcol_name] | MODIFY [COLUMN]column_definition[FIRST | AFTERcol_name] | DROP [COLUMN]col_name| DROP PRIMARY KEY | DROP {INDEX|KEY}index_name| DROP FOREIGN KEYfk_symbol| DISABLE KEYS | ENABLE KEYS | RENAME [TO]new_tbl_name| ORDER BYcol_name[,col_name] ... | CONVERT TO CHARACTER SETcharset_name[COLLATEcollation_name] | [DEFAULT] CHARACTER SETcharset_name[COLLATEcollation_name] | DISCARD TABLESPACE | IMPORT TABLESPACEindex_col_name:col_name[(length)] [ASC | DESC]index_type: USING {BTREE | HASH}
ALTER TABLE enables you to change the structure of an existing table. For example, you can add or delete columns, create or destroy indexes, change the type of existing columns, or rename columns or the table itself. You can also change the comment for the table and type of the table.
The syntax for many of the allowable alterations is similar to clauses of the CREATE TABLE statement. See Section 13.1.5, “CREATE TABLE Syntax”, for more information.
Some operations may result in warnings if attempted on a table for which the storage engine does not support the operation. These warnings can be displayed with SHOW WARNINGS. See Section 13.5.4.28, “SHOW WARNINGS Syntax”.
If you use ALTER TABLE to change a column specification but DESCRIBE indicates that your column was not changed, it is possible that MySQL ignored your modification for one of the reasons described in Section 13.1.5.1, “Silent Column Specification Changes”. tbl_name
In most cases, ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, and then the original table is deleted and the new one is renamed. While ALTER TABLE is executing, the original table is readable by other clients. Updates and writes to the table are stalled until the new table is ready, and then are automatically redirected to the new table without any failed updates.
If you use ALTER TABLE without any other options, MySQL simply renames any files that correspond to the table tbl_name RENAME TO new_tbl_nametbl_name. (You can also use the RENAME TABLE statement to rename tables. See Section 13.1.9, “RENAME TABLE Syntax”.) Any privileges granted specifically for the renamed table are not migrated to the new name. They must be changed manually.
If you use any option to ALTER TABLE other than RENAME, MySQL always creates a temporary table, even if the data wouldn't strictly need to be copied (such as when you change the name of a column). For MyISAM tables, you can speed up the index re-creation operation (which is the slowest part of the alteration process) by setting the myisam_sort_buffer_size system variable to a high value.
To use ALTER TABLE, you need ALTER, INSERT, and CREATE privileges for the table.
IGNORE is a MySQL extension to standard SQL. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table or if warnings occur when strict mode is enabled. If IGNORE is not specified, the copy is aborted and rolled back if duplicate-key errors occur. If IGNORE is specified, only the first row is used of rows with duplicates on a unique key, The other conflicting rows are deleted. Incorrect values are truncated to the closest matching acceptable value.
table_option signifies a table option of the kind that can be used in the CREATE TABLE statement. (Section 13.1.5, “CREATE TABLE Syntax”, lists all table options.) This includes options such as ENGINE, AUTO_INCREMENT, and AVG_ROW_LENGTH. However, ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.
For example, to convert a table to be an InnoDB table, use this statement:
ALTER TABLE t1 ENGINE = InnoDB;
The outcome of attempting to change a table's storage engine is affected by whether the desired storage engine is available and the setting of the NO_ENGINE_SUBSTITUTION SQL mode, as described in Section 5.2.6, “SQL Modes”.
As of MySQL 5.0.23, to prevent inadvertent loss of data, ALTER TABLE cannot be used to change the storage engine of a table to MERGE or BLACKHOLE.
To change the value of the AUTO_INCREMENT counter to be used for new rows, do this:
ALTER TABLE t2 AUTO_INCREMENT = value; You cannot reset the counter to a value less than or equal to any that have already been used. For MyISAM, if the value is less than or equal to the maximum value currently in the AUTO_INCREMENT column, the value is reset to the current maximum plus one. For InnoDB, you can use ALTER TABLE ... AUTO_INCREMENT = as of MySQL 5.0.3, but if the value is less than the current maximum value in the column, no error message is given and the current sequence value is not changed.value
You can issue multiple ADD, ALTER, DROP, and CHANGE clauses in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which allows only one of each clause per ALTER TABLE statement. For example, to drop multiple columns in a single statement, do this:
ALTER TABLE t2 DROP COLUMN c, DROP COLUMN d;
CHANGE , col_nameDROP , and col_nameDROP INDEX are MySQL extensions to standard SQL.
MODIFY is an Oracle extension to ALTER TABLE.
The word COLUMN is optional and can be omitted.
column_definition clauses use the same syntax for ADD and CHANGE as for CREATE TABLE. Note that this syntax includes the column name, not just its data type. See Section 13.1.5, “CREATE TABLE Syntax”.
You can rename a column using a CHANGE clause. To do so, specify the old and new column names and the type that the column currently has. For example, to rename an old_col_name column_definitionINTEGER column from a to b, you can do this:
ALTER TABLE t1 CHANGE a b INTEGER;
If you want to change a column's type but not the name, CHANGE syntax still requires an old and new column name, even if they are the same. For example:
ALTER TABLE t1 CHANGE b b BIGINT NOT NULL;
You can also use MODIFY to change a column's type without renaming it:
ALTER TABLE t1 MODIFY b BIGINT NOT NULL;
If you use CHANGE or MODIFY to shorten a column for which an index exists on the column, and the resulting column length is less than the index length, MySQL shortens the index automatically.
When you change a data type using CHANGE or MODIFY, MySQL tries to convert existing column values to the new type as well as possible.
To add a column at a specific position within a table row, use FIRST or AFTER . The default is to add the column last. You can also use col_nameFIRST and AFTER in CHANGE or MODIFY operations.
ALTER ... SET DEFAULT or ALTER ... DROP DEFAULT specify a new default value for a column or remove the old default value, respectively. If the old default is removed and the column can be NULL, the new default is NULL. If the column cannot be NULL, MySQL assigns a default value, as described in Section 11.1.4, “Data Type Default Values”.
DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.7, “DROP INDEX Syntax”.
If columns are dropped from a table, the columns are also removed from any index of which they are a part. If all columns that make up an index are dropped, the index is dropped as well.
If a table contains only one column, the column cannot be dropped. If what you intend is to remove the table, use DROP TABLE instead.
DROP PRIMARY KEY drops the primary index. Note: In older versions of MySQL, if no primary index existed, DROP PRIMARY KEY would drop the first UNIQUE index in the table. This is not the case in MySQL 5.0, where trying to use DROP PRIMARY KEY on a table with no primary key results in an error.
If you add a UNIQUE INDEX or PRIMARY KEY to a table, it is stored before any non-unique index so that MySQL can detect duplicate keys as early as possible.
Some storage engines allow you to specify an index type when creating an index. The syntax for the index_type specifier is USING . For details about type_nameUSING, see Section 13.1.4, “CREATE INDEX Syntax”.
After an ALTER TABLE statement, it may be necessary to run ANALYZE TABLE to update index cardinality information. See Section 13.5.4.13, “SHOW INDEX Syntax”.
ORDER BY enables you to create the new table with the rows in a specific order. Note that the table does not remain in this order after inserts and deletes. This option is useful primarily when you know that you are mostly to query the rows in a certain order most of the time. By using this option after major changes to the table, you might be able to get higher performance. In some cases, it might make sorting easier for MySQL if the table is in order by the column that you want to order it by later.
ORDER BY syntax allows for one or more column names to be specified for sorting, each of which optionally can be followed by ASC or DESC to indicate ascending or descending sort order, respectively. The default is ascending order. Only column names are allowed as sort criteria; arbitrary expressions are not allowed.
If you use ALTER TABLE on a MyISAM table, all non-unique indexes are created in a separate batch (as for REPAIR TABLE). This should make ALTER TABLE much faster when you have many indexes.
This feature can be activated explicitly. ALTER TABLE ... DISABLE KEYS tells MySQL to stop updating non-unique indexes for a MyISAM table. ALTER TABLE ... ENABLE KEYS then should be used to re-create missing indexes. MySQL does this with a special algorithm that is much faster than inserting keys one by one, so disabling keys before performing bulk insert operations should give a considerable speedup. Using ALTER TABLE ... DISABLE KEYS requires the INDEX privilege in addition to the privileges mentioned earlier.
While the non-unique indexes are disabled, they are ignored for statements such as SELECT and EXPLAIN that otherwise would use them.
The FOREIGN KEY and REFERENCES clauses are supported by the InnoDB storage engine, which implements ADD [CONSTRAINT [. See Section 14.2.6.4, “symbol]] FOREIGN KEY (...) REFERENCES ... (...)FOREIGN KEY Constraints”. For other storage engines, the clauses are parsed but ignored. The CHECK clause is parsed but ignored by all storage engines. See Section 13.1.5, “CREATE TABLE Syntax”. The reason for accepting but ignoring syntax clauses is for compatibility, to make it easier to port code from other SQL servers, and to run applications that create tables with references. See Section 1.9.5, “MySQL Differences from Standard SQL”.
You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. You must use separate statements.
InnoDB supports the use of ALTER TABLE to drop foreign keys:
ALTER TABLEtbl_nameDROP FOREIGN KEYfk_symbol;
You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. You must use separate statements.
For more information, see Section 14.2.6.4, “FOREIGN KEY Constraints”.
Pending INSERT DELAYED statements are lost if a table is write locked and ALTER TABLE is used to modify the table structure.
If you want to change the table default character set and all character columns (CHAR, VARCHAR, TEXT) to a new character set, use a statement like this:
ALTER TABLEtbl_nameCONVERT TO CHARACTER SETcharset_name;
Warning: The preceding operation converts column values between the character sets. This is not what you want if you have a column in one character set (like latin1) but the stored values actually use some other, incompatible character set (like utf8). In this case, you have to do the following for each such column:
ALTER TABLE t1 CHANGE c1 c1 BLOB; ALTER TABLE t1 CHANGE c1 c1 TEXT CHARACTER SET utf8;
The reason this works is that there is no conversion when you convert to or from BLOB columns.
If you specify CONVERT TO CHARACTER SET binary, the CHAR, VARCHAR, and TEXT columns are converted to their corresponding binary string types (BINARY, VARBINARY, BLOB). This means that the columns no longer will have a character set and a subsequent CONVERT TO operation will not apply to them.
To change only the default character set for a table, use this statement:
ALTER TABLEtbl_nameDEFAULT CHARACTER SETcharset_name;
The word DEFAULT is optional. The default character set is the character set that is used if you do not specify the character set for a new column which you add to a table (for example, with ALTER TABLE ... ADD column).
For an InnoDB table that is created with its own tablespace in an .ibd file, that file can be discarded and imported. To discard the .ibd file, use this statement:
ALTER TABLE tbl_name DISCARD TABLESPACE; This deletes the current .ibd file, so be sure that you have a backup first. Attempting to access the table while the tablespace file is discarded results in an error.
To import the backup .ibd file back into the table, copy it into the database directory, and then issue this statement:
ALTER TABLE tbl_name IMPORT TABLESPACE; With the mysql_info() C API function, you can find out how many rows were copied, and (when IGNORE is used) how many rows were deleted due to duplication of unique key values. See Section 22.2.3.35, “mysql_info()”.
Here are some examples that show uses of ALTER TABLE. Begin with a table t1 that is created as shown here:
CREATE TABLE t1 (a INTEGER,b CHAR(10));
To rename the table from t1 to t2:
ALTER TABLE t1 RENAME t2;
To change column a from INTEGER to TINYINT NOT NULL (leaving the name the same), and to change column b from CHAR(10) to CHAR(20) as well as renaming it from b to c:
ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);
To add a new TIMESTAMP column named d:
ALTER TABLE t2 ADD d TIMESTAMP;
To add indexes on column d and on column a:
ALTER TABLE t2 ADD INDEX (d), ADD INDEX (a);
To remove column c:
ALTER TABLE t2 DROP COLUMN c;
To add a new AUTO_INCREMENT integer column named c:
ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT, ADD PRIMARY KEY (c);
Note that we indexed c (as a PRIMARY KEY), because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because primary key columns cannot be NULL.
When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically. For MyISAM tables, you can set the first sequence number by executing SET INSERT_ID= before valueALTER TABLE or by using the AUTO_INCREMENT= table option. See Section 13.5.3, “valueSET Syntax”.
With MyISAM tables, if you do not change the AUTO_INCREMENT column, the sequence number is not affected. If you drop an AUTO_INCREMENT column and then add another AUTO_INCREMENT column, the numbers are resequenced beginning with 1.
When replication is used, adding an AUTO_INCREMENT column to a table might not produce the same ordering of the rows on the slave and the master. This occurs because the order in which the rows are numbered depends on the specific storage engine used for the table and the order in which the rows were inserted. If it is important to have the same order on the master and slave, the rows must be ordered before assigning an AUTO_INCREMENT number. Assuming that you want to add an AUTO_INCREMENT column to the table t1, the following statements produce a new table t2 identical to t1 but with an AUTO_INCREMENT column:
CREATE TABLE t2 (id INT AUTO_INCREMENT PRIMARY KEY) SELECT * FROM t1 ORDER BY col1, col2;
This assumes that the table t1 has columns col1 and col2.
This set of statements will also produce a new table t2 identical to t1, with the addition of an AUTO_INCREMENT column:
CREATE TABLE t2 LIKE t1; ALTER TABLE T2 ADD id INT AUTO_INCREMENT PRIMARY KEY; INSERT INTO t2 SELECT * FROM t1 ORDER BY col1, col2;
Important: To guarantee the same ordering on both master and slave, all columns of t1 must be referenced in the ORDER BY clause.
Regardless of the method used to create and populate the copy having the AUTO_INCREMENT column, the final step is to drop the original table and then rename the copy:
DROP t1; ALTER TABLE t2 RENAME t1;
CREATE {DATABASE | SCHEMA} [IF NOT EXISTS] db_name
[create_specification [create_specification] ...]
create_specification:
[DEFAULT] CHARACTER SET charset_name
| [DEFAULT] COLLATE collation_name CREATE DATABASE creates a database with the given name. To use this statement, you need the CREATE privilege for the database. CREATE SCHEMA is a synonym for CREATE DATABASE as of MySQL 5.0.2.
An error occurs if the database exists and you did not specify IF NOT EXISTS.
create_specification options specify database characteristics. Database characteristics are stored in the db.opt file in the database directory. The CHARACTER SET clause specifies the default database character set. The COLLATE clause specifies the default database collation. Chapter 10, Character Set Support, discusses character set and collation names.
A database in MySQL is implemented as a directory containing files that correspond to tables in the database. Because there are no tables in a database when it is initially created, the CREATE DATABASE statement creates only a directory under the MySQL data directory and the db.opt file. Rules for allowable database names are given in Section 9.2, “Database, Table, Index, Column, and Alias Names”.
If you manually create a directory under the data directory (for example, with mkdir), the server considers it a database directory and it shows up in the output of SHOW DATABASES.
You can also use the mysqladmin program to create databases. See Section 8.10, “mysqladmin — Client for Administering a MySQL Server”.
CREATE [UNIQUE|FULLTEXT|SPATIAL] INDEXindex_name[index_type] ONtbl_name(index_col_name,...)index_col_name:col_name[(length)] [ASC | DESC]index_type: USING {BTREE | HASH}
CREATE INDEX is mapped to an ALTER TABLE statement to create indexes. See Section 13.1.2, “ALTER TABLE Syntax”. CREATE INDEX cannot be used to create a PRIMARY KEY; use ALTER TABLE instead. For more information about indexes, see Section 7.4.5, “How MySQL Uses Indexes”.
Normally, you create all indexes on a table at the time the table itself is created with CREATE TABLE. See Section 13.1.5, “CREATE TABLE Syntax”. CREATE INDEX enables you to add indexes to existing tables.
A column list of the form (col1,col2,...) creates a multiple-column index. Index values are formed by concatenating the values of the given columns.
Indexes can be created that use only the leading part of column values, using col_name(length)
Prefixes can be specified for CHAR, VARCHAR, BINARY, and VARBINARY columns.
BLOB and TEXT columns also can be indexed, but a prefix length must be given.
Prefix lengths are given in characters for non-binary string types and in bytes for binary string types. That is, index entries consist of the first length characters of each column value for CHAR, VARCHAR, and TEXT columns, and the first length bytes of each column value for BINARY, VARBINARY, and BLOB columns.
For spatial columns, prefix values can be given as described later in this section.
The statement shown here creates an index using the first 10 characters of the name column:
CREATE INDEX part_of_name ON customer (name(10));
If names in the column usually differ in the first 10 characters, this index should not be much slower than an index created from the entire name column. Also, using column prefixes for indexes can make the index file much smaller, which could save a lot of disk space and might also speed up INSERT operations.
Prefix lengths are storage engine-dependent (for example, a prefix can be up to 1000 bytes long for MyISAM tables, 767 bytes for InnoDB tables). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE INDEX statements is interpreted as number of characters for non-binary data types (CHAR, VARCHAR, TEXT). Take this into account when specifying a prefix length for a column that uses a multi-byte character set. For example, utf8 columns require up to three index bytes per character.
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within the prefix.
MySQL Enterprise. Lack of proper indexes can greatly reduce performance. Subscribe to the MySQL Network Monitoring and Advisory Service for notification of inefficient use of indexes. For more information see http://www.mysql.com/products/enterprise/advisors.html.
FULLTEXT indexes are supported only for MyISAM tables and can include only CHAR, VARCHAR, and TEXT columns. Indexing always happens over the entire column; column prefix indexing is not supported and any prefix length is ignored if specified. See Section 12.8, “Full-Text Search Functions”, for details of operation.
The MyISAM, InnoDB, NDB, BDB, and ARCHIVE storage engines support spatial columns such as (POINT and GEOMETRY. (Chapter 16, Spatial Extensions, describes the spatial data types.) However, support for spatial column indexing varies among engines. Spatial and non-spatial indexes are available according to the following rules.
Spatial indexes (created using SPATIAL INDEX):
Available only for MyISAM tables. Specifying a SPATIAL INDEX for other storage engines results in an error.
Indexed columns must be NOT NULL.
In MySQL 5.0, the full width of each column is indexed by default, but column prefix lengths are allowed. However, as of MySQL 5.0.40, the length is not displayed in SHOW CREATE TABLE output. mysqldump uses that statement. As of that version, if a table with SPATIAL indexes containing prefixed columns is dumped and reloaded, the index is created with no prefixes. (The full column width of each column is indexed.)
Non-spatial indexes (created with INDEX, UNIQUE, or PRIMARY KEY):
Allowed for any storage engine that supports spatial columns except ARCHIVE.
Columns can be NULL unless the index is a primary key.
For each spatial column in a non-SPATIAL index except POINT columns, a column prefix length must be specified. (This is the same requirement as for indexed BLOB columns.) The prefix length is given in bytes.
The index type for a non-SPATIAL index depends on the storage engine. Currently, B-tree is used.
In MySQL 5.0:
You can add an index on a column that can have NULL values only if you are using the MyISAM, InnoDB, BDB, or MEMORY storage engine.
You can add an index on a BLOB or TEXT column only if you are using the MyISAM, BDB, or InnoDB storage engine.
An index_col_name specification can end with ASC or DESC. These keywords are allowed for future extensions for specifying ascending or descending index value storage. Currently, they are parsed but ignored; index values are always stored in ascending order.
Some storage engines allow you to specify an index type when creating an index. The allowable index type values supported by different storage engines are shown in the following table. Where multiple index types are listed, the first one is the default when no index type specifier is given.
| Storage Engine | Allowable Index Types |
MyISAM | BTREE |
InnoDB | BTREE |
MEMORY/HEAP | HASH, BTREE |
NDB | HASH |
If you specify an index type that is not legal for a given storage engine, but there is another index type available that the engine can use without affecting query results, the engine uses the available type.
Examples:
CREATE TABLE lookup (id INT) ENGINE = MEMORY; CREATE INDEX id_index USING BTREE ON lookup (id);
For indexes on NDB table columns, the USING clause can be specified only for a unique index or primary key. In such cases, the USING HASH clause prevents the creation of an implicit ordered index. Without USING HASH, a statement defining a unique index or primary key automatically results in the creation of a HASH index in addition to the ordered index, both of which index the same set of columns.
TYPE is recognized as a synonym for type_nameUSING . However, type_nameUSING is the preferred form.
CREATE [TEMPORARY] TABLE [IF NOT EXISTS]tbl_name(create_definition,...) [table_option...]
Or:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS]tbl_name[(create_definition,...)] [table_option...]select_statement
Or:
CREATE [TEMPORARY] TABLE [IF NOT EXISTS]tbl_name{ LIKEold_tbl_name| (LIKEold_tbl_name) }create_definition:column_definition| [CONSTRAINT [symbol]] PRIMARY KEY [index_type] (index_col_name,...) | {INDEX|KEY} [index_name] [index_type] (index_col_name,...) | [CONSTRAINT [symbol]] UNIQUE [INDEX|KEY] [index_name] [index_type] (index_col_name,...) | {FULLTEXT|SPATIAL} [INDEX|KEY] [index_name] (index_col_name,...) | [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (index_col_name,...) [reference_definition] | CHECK (expr)column_definition:col_namedata_type[NOT NULL | NULL] [DEFAULTdefault_value] [AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY] [COMMENT 'string'] [reference_definition]data_type: BIT[(length)] | TINYINT[(length)] [UNSIGNED] [ZEROFILL] | SMALLINT[(length)] [UNSIGNED] [ZEROFILL] | MEDIUMINT[(length)] [UNSIGNED] [ZEROFILL] | INT[(length)] [UNSIGNED] [ZEROFILL] | INTEGER[(length)] [UNSIGNED] [ZEROFILL] | BIGINT[(length)] [UNSIGNED] [ZEROFILL] | REAL[(length,decimals)] [UNSIGNED] [ZEROFILL] | DOUBLE[(length,decimals)] [UNSIGNED] [ZEROFILL] | FLOAT[(length,decimals)] [UNSIGNED] [ZEROFILL] | DECIMAL(length,decimals) [UNSIGNED] [ZEROFILL] | NUMERIC(length,decimals) [UNSIGNED] [ZEROFILL] | DATE | TIME | TIMESTAMP | DATETIME | YEAR | CHAR(length) [CHARACTER SETcharset_name] [COLLATEcollation_name] | VARCHAR(length) [CHARACTER SETcharset_name] [COLLATEcollation_name] | BINARY(length) | VARBINARY(length) | TINYBLOB | BLOB | MEDIUMBLOB | LONGBLOB | TINYTEXT [BINARY] [CHARACTER SETcharset_name] [COLLATEcollation_name] | TEXT [BINARY] [CHARACTER SETcharset_name] [COLLATEcollation_name] | MEDIUMTEXT [BINARY] [CHARACTER SETcharset_name] [COLLATEcollation_name] | LONGTEXT [BINARY] [CHARACTER SETcharset_name] [COLLATEcollation_name] | ENUM(value1,value2,value3,...) [CHARACTER SETcharset_name] [COLLATEcollation_name] | SET(value1,value2,value3,...) [CHARACTER SETcharset_name] [COLLATEcollation_name] |spatial_typeindex_col_name:col_name[(length)] [ASC | DESC]index_type: USING {BTREE | HASH}reference_definition: REFERENCEStbl_name[(index_col_name,...)] [MATCH FULL | MATCH PARTIAL | MATCH SIMPLE] [ON DELETEreference_option] [ON UPDATEreference_option]reference_option: RESTRICT | CASCADE | SET NULL | NO ACTIONtable_option: {ENGINE|TYPE} [=]engine_name| AUTO_INCREMENT [=]value| AVG_ROW_LENGTH [=]value| [DEFAULT] CHARACTER SETcharset_name| CHECKSUM [=] {0 | 1} | COLLATEcollation_name| COMMENT [=] 'string' | CONNECTION [=] 'connect_string' | DATA DIRECTORY [=] 'absolute path to directory' | DELAY_KEY_WRITE [=] {0 | 1} | INDEX DIRECTORY [=] 'absolute path to directory' | INSERT_METHOD [=] { NO | FIRST | LAST } | MAX_ROWS [=]value| MIN_ROWS [=]value| PACK_KEYS [=] {0 | 1 | DEFAULT} | PASSWORD [=] 'string' | ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT} | UNION [=] (tbl_name[,tbl_name]...)select_statement:[IGNORE | REPLACE] [AS] SELECT ... (Some legal select statement)
CREATE TABLE creates a table with the given name. You must have the CREATE privilege for the table.
Rules for allowable table names are given in Section 9.2, “Database, Table, Index, Column, and Alias Names”. By default, the table is created in the default database. An error occurs if the table exists, if there is no default database, or if the database does not exist.
The table name can be specified as db_name.tbl_name to create the table in a specific database. This works regardless of whether there is a default database, assuming that the database exists. If you use quoted identifiers, quote the database and table names separately. For example, write `mydb`.`mytbl`, not `mydb.mytbl`.
You can use the TEMPORARY keyword when creating a table. A TEMPORARY table is visible only to the current connection, and is dropped automatically when the connection is closed. This means that two different connections can use the same temporary table name without conflicting with each other or with an existing non-TEMPORARY table of the same name. (The existing table is hidden until the temporary table is dropped.) To create temporary tables, you must have the CREATE TEMPORARY TABLES privilege.
Note: CREATE TABLE does not automatically commit the current active transaction if you use the TEMPORARY keyword.
The keywords IF NOT EXISTS prevent an error from occurring if the table exists. However, there is no verification that the existing table has a structure identical to that indicated by the CREATE TABLE statement. Note: If you use IF NOT EXISTS in a CREATE TABLE ... SELECT statement, any rows selected by the SELECT part are inserted regardless of whether the table already exists.
MySQL represents each table by an .frm table format (definition) file in the database directory. The storage engine for the table might create other files as well. In the case of MyISAM tables, the storage engine creates data and index files. Thus, for each MyISAM table tbl_name, there are three disk files:
| File | Purpose |
| Table format (definition) file |
| Data file |
| Index file |
Chapter 14, Storage Engines, describes what files each storage engine creates to represent tables.
data_type represents the data type is a column definition. spatial_type represents a spatial data type. The data type syntax shown is representative only. For a full description of the syntax available for specfiying column data types, as well as information about the properties of each type, see Chapter 11, Data Types, and Chapter 16, Spatial Extensions.
Some attributes do not apply to all data types. AUTO_INCREMENT applies only to integer types. DEFAULT does not apply to the BLOB or TEXT types.
If neither NULL nor NOT NULL is specified, the column is treated as though NULL had been specified.
An integer column can have the additional attribute AUTO_INCREMENT. When you insert a value of NULL (recommended) or 0 into an indexed AUTO_INCREMENT column, the column is set to the next sequence value. Typically this is value+1value is the largest value for the column currently in the table. AUTO_INCREMENT sequences begin with 1.
To retrieve an AUTO_INCREMENT value after inserting a row, use the LAST_INSERT_ID() SQL function or the mysql_insert_id() C API function. See Section 12.10.3, “Information Functions”, and Section 22.2.3.37, “mysql_insert_id()”.
If the NO_AUTO_VALUE_ON_ZERO SQL mode is enabled, you can store 0 in AUTO_INCREMENT columns as 0 without generating a new sequence value. See Section 5.2.6, “SQL Modes”.
Note: There can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot have a DEFAULT value. An AUTO_INCREMENT column works properly only if it contains only positive values. Inserting a negative number is regarded as inserting a very large positive number. This is done to avoid precision problems when numbers “wrap” over from positive to negative and also to ensure that you do not accidentally get an AUTO_INCREMENT column that contains 0.
For MyISAM and BDB tables, you can specify an AUTO_INCREMENT secondary column in a multiple-column key. See Section 3.6.9, “Using AUTO_INCREMENT”.
To make MySQL compatible with some ODBC applications, you can find the AUTO_INCREMENT value for the last inserted row with the following query:
SELECT * FROMtbl_nameWHEREauto_colIS NULL
For information about InnoDB and AUTO_INCREMENT, see Section 14.2.6.3, “How AUTO_INCREMENT Columns Work in InnoDB”.
Character data types (CHAR, VARCHAR, TEXT) can include CHARACTER SET and COLLATE attributes to specify the character set and collation for the column. For details, see Chapter 10, Character Set Support. CHARSET is a synonym for CHARACTER SET. Example:
CREATE TABLE t (c CHAR(20) CHARACTER SET utf8 COLLATE utf8_bin);
MySQL 5.0 interprets length specifications in character column definitions in characters. (Versions before MySQL 4.1 interpreted them in bytes.) Lengths for BINARY and VARBINARY are in bytes.
The DEFAULT clause specifies a default value for a column. With one exception, the default value must be a constant; it cannot be a function or an expression. This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE. The exception is that you can specify CURRENT_TIMESTAMP as the default for a TIMESTAMP column. See Section 11.3.1.1, “TIMESTAMP Properties as of MySQL 4.1”.
If a column definition includes no explicit DEFAULT value, MySQL determines the default value as described in Section 11.1.4, “Data Type Default Values”.
BLOB and TEXT columns cannot be assigned a default value.
A comment for a column can be specified with the COMMENT option, up to 255 characters long. The comment is displayed by the SHOW CREATE TABLE and SHOW FULL COLUMNS statements.
KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. This constraint does not apply to NULL values except for the BDB storage engine. For other engines, a UNIQUE index allows multiple NULL values for columns that can contain NULL.
A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently). A table can have only one PRIMARY KEY. If you do not have a PRIMARY KEY and an application asks for the PRIMARY KEY in your tables, MySQL returns the first UNIQUE index that has no NULL columns as the PRIMARY KEY.
In InnoDB tables, having a long PRIMARY KEY wastes a lot of space. (See Section 14.2.13, “InnoDB Table and Index Structures”.)
In the created table, a PRIMARY KEY is placed first, followed by all UNIQUE indexes, and then the non-unique indexes. This helps the MySQL optimizer to prioritize which index to use and also more quickly to detect duplicated UNIQUE keys.
A PRIMARY KEY can be a multiple-column index. However, you cannot create a multiple-column index using the PRIMARY KEY key attribute in a column specification. Doing so only marks that single column as primary. You must use a separate PRIMARY KEY( clause.index_col_name, ...)
If a PRIMARY KEY or UNIQUE index consists of only one column that has an integer type, you can also refer to the column as _rowid in SELECT statements.
In MySQL, the name of a PRIMARY KEY is PRIMARY. For other indexes, if you do not assign a name, the index is assigned the same name as the first indexed column, with an optional suffix (_2, _3, ...) to make it unique. You can see index names for a table using SHOW INDEX FROM . See Section 13.5.4.13, “tbl_nameSHOW INDEX Syntax”.
Some storage engines allow you to specify an index type when creating an index. The syntax for the index_type specifier is USING . type_name
Example:
CREATE TABLE lookup (id INT, INDEX USING BTREE (id)) ENGINE = MEMORY;
For details about USING, see Section 13.1.4, “CREATE INDEX Syntax”.
For more information about indexes, see Section 7.4.5, “How MySQL Uses Indexes”.
In MySQL 5.0, only the MyISAM, InnoDB, BDB, and MEMORY storage engines support indexes on columns that can have NULL values. In other cases, you must declare indexed columns as NOT NULL or an error results.
For CHAR, VARCHAR, BINARY, and VARBINARY columns, indexes can be created that use only the leading part of column values, using col_name(length)BLOB and TEXT columns also can be indexed, but a prefix length must be given. Prefix lengths are given in characters for non-binary string types and in bytes for binary string types. That is, index entries consist of the first length characters of each column value for CHAR, VARCHAR, and TEXT columns, and the first length bytes of each column value for BINARY, VARBINARY, and BLOB columns. Indexing only a prefix of column values like this can make the index file much smaller. See Section 7.4.3, “Column Indexes”.
Only the MyISAM, BDB, and InnoDB storage engines support indexing on BLOB and TEXT columns. For example:
CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10)));
Prefixes can be up to 1000 bytes long (767 bytes for InnoDB tables). Note that prefix limits are measured in bytes, whereas the prefix length in CREATE TABLE statements is interpreted as number of characters for non-binary data types (CHAR, VARCHAR, TEXT). Take this into account when specifying a prefix length for a column that uses a multi-byte character set.
An index_col_name specification can end with ASC or DESC. These keywords are allowed for future extensions for specifying ascending or descending index value storage. Currently, they are parsed but ignored; index values are always stored in ascending order.
When you use ORDER BY or GROUP BY on a TEXT or BLOB column in a SELECT, the server sorts values using only the initial number of bytes indicated by the max_sort_length system variable. See Section 11.4.3, “The BLOB and TEXT Types”.
You can create special FULLTEXT indexes, which are used for full-text searches. Only the MyISAM storage engine supports FULLTEXT indexes. They can be created only from CHAR, VARCHAR, and TEXT columns. Indexing always happens over the entire column; column prefix indexing is not supported and any prefix length is ignored if specified. See Section 12.8, “Full-Text Search Functions”, for details of operation.
You can create SPATIAL indexes on spatial data types. Spatial types are supported only for MyISAM tables and indexed columns must be declared as NOT NULL. See Chapter 16, Spatial Extensions.
InnoDB tables support checking of foreign key constraints. See Section 14.2, “The InnoDB Storage Engine”. Note that the FOREIGN KEY syntax in InnoDB is more restrictive than the syntax presented for the CREATE TABLE statement at the beginning of this section: The columns of the referenced table must always be explicitly named. InnoDB supports both ON DELETE and ON UPDATE actions on foreign keys. For the precise syntax, see Section 14.2.6.4, “FOREIGN KEY Constraints”.
For other storage engines, MySQL Server parses and ignores the FOREIGN KEY and REFERENCES syntax in CREATE TABLE statements. The CHECK clause is parsed but ignored by all storage engines. See Section 1.9.5.5, “Foreign Keys”.
For MyISAM tables, each NULL column takes one bit extra, rounded up to the nearest byte. The maximum row length in bytes can be calculated as follows:
row length = 1
+ (sum of column lengths)
+ (number of NULL columns + delete_flag + 7)/8
+ (number of variable-length columns) delete_flag is 1 for tables with static row format. Static tables use a bit in the row record for a flag that indicates whether the row has been deleted. delete_flag is 0 for dynamic tables because the flag is stored in the dynamic row header.
These calculations do not apply for InnoDB tables, for which storage size is no different for NULL columns than for NOT NULL columns.
The ENGINE table option specifies the storage engine for the table. TYPE is a synonym, but ENGINE is the preferred option name.
The ENGINE table option takes the storage engine names shown in the following table.
| Storage Engine | Description |
ARCHIVE | The archiving storage engine. See Section 14.8, “The ARCHIVE Storage Engine”. |
BDB | Transaction-safe tables with page locking. Also known as BerkeleyDB. See Section 14.5, “The BDB (BerkeleyDB) Storage Engine”. |
CSV | Tables that store rows in comma-separated values format. See Section 14.9, “The CSV Storage Engine”. |
EXAMPLE | An example engine. See Section 14.6, “The EXAMPLE Storage Engine”. |
FEDERATED | Storage engine that accesses remote tables. See Section 14.7, “The FEDERATED Storage Engine”. |
HEAP | This is a synonym for MEMORY. |
ISAM (OBSOLETE) | Not available in MySQL 5.0. If you are upgrading to MySQL 5.0 from a previous version, you should convert any existing ISAM tables to MyISAM before performing the upgrade. |
InnoDB | Transaction-safe tables with row locking and foreign keys. See Section 14.2, “The InnoDB Storage Engine”. |
MEMORY | The data for this storage engine is stored only in memory. See Section 14.4, “The MEMORY (HEAP) Storage Engine”. |
MERGE | A collection of MyISAM tables used as one table. Also known as MRG_MyISAM. See Section 14.3, “The MERGE Storage Engine”. |
MyISAM | The binary portable storage engine that is the default storage engine used by MySQL. See Section 14.1, “The MyISAM Storage Engine”. |
NDBCLUSTER | Clustered, fault-tolerant, memory-based tables. Also known as NDB. See Chapter 15, MySQL Cluster. |
If a storage engine is specified that is not available, MySQL uses the default engine instead. Normally, this is MyISAM. For example, if a table definition includes the ENGINE=BDB option but the MySQL server does not support BDB tables, the table is created as a MyISAM table. This makes it possible to have a replication setup where you have transactional tables on the master but tables created on the slave are non-transactional (to get more speed). In MySQL 5.0, a warning occurs if the storage engine specification is not honored.
Engine substitution can be controlled by the setting of the NO_ENGINE_SUBSTITUTION SQL mode, as described in Section 5.2.6, “SQL Modes”.
The other table options are used to optimize the behavior of the table. In most cases, you do not have to specify any of them. These options apply to all storage engines unless otherwise indicated. Options that do not apply to a given storage engine may be accepted and remembered as part of the table definition. Such options then apply if you later use ALTER TABLE to convert the table to use a different storage engine.
AUTO_INCREMENT
The initial AUTO_INCREMENT value for the table. In MySQL 5.0, this works for MyISAM and MEMORY tables. It is also supported for InnoDB as of MySQL 5.0.3. To set the first auto-increment value for engines that do not support the AUTO_INCREMENT table option, insert a “dummy” row with a value one less than the desired value after creating the table, and then delete the dummy row.
For engines that support the AUTO_INCREMENT table option in CREATE TABLE statements, you can also use ALTER TABLE to reset the tbl_name AUTO_INCREMENT = NAUTO_INCREMENT value. The value cannot be set lower than the maximum value currently in the column.
AVG_ROW_LENGTH
An approximation of the average row length for your table. You need to set this only for large tables with variable-size rows.
When you create a MyISAM table, MySQL uses the product of the MAX_ROWS and AVG_ROW_LENGTH options to decide how big the resulting table is. If you don't specify either option, the maximum size for a table is 256TB of data by default (4GB before MySQL 5.0.6). (If your operating system does not support files that large, table sizes are constrained by the file size limit.) If you want to keep down the pointer sizes to make the index smaller and faster and you don't really need big files, you can decrease the default pointer size by setting the myisam_data_pointer_size system variable, which was added in MySQL 4.1.2. (See Section 5.2.3, “System Variables”.) If you want all your tables to be able to grow above the default limit and are willing to have your tables slightly slower and larger than necessary, you can increase the default pointer size by setting this variable.
[DEFAULT] CHARACTER SET
Specify a default character set for the table. CHARSET is a synonym for CHARACTER SET.
CHECKSUM
Set this to 1 if you want MySQL to maintain a live checksum for all rows (that is, a checksum that MySQL updates automatically as the table changes). This makes the table a little slower to update, but also makes it easier to find corrupted tables. The CHECKSUM TABLE statement reports the checksum. (MyISAM only.)
COLLATE
Specify a default collation for the table.
COMMENT
A comment for the table, up to 60 characters long.
CONNECTION
The connection string for a FEDERATED table. This option is available as of MySQL 5.0.13; before that, use a COMMENT option for the connection string.
DATA DIRECTORY, INDEX DIRECTORY
By using DATA DIRECTORY=' or directory'INDEX DIRECTORY=' you can specify where the directory'MyISAM storage engine should put a table's data file and index file. The directory must be the full pathname to the directory, not a relative path.
These options work only when you are not using the --skip-symbolic-links option. Your operating system must also have a working, thread-safe realpath() call. See Section 7.6.1.2, “Using Symbolic Links for Tables on Unix”, for more complete information.
DELAY_KEY_WRITE
Set this to 1 if you want to delay key updates for the table until the table is closed. See the description of the delay_key_write system variable in Section 5.2.3, “System Variables”. (MyISAM only.)
INSERT_METHOD
If you want to insert data into a MERGE table, you must specify with INSERT_METHOD the table into which the row should be inserted. INSERT_METHOD is an option useful for MERGE tables only. Use a value of FIRST or LAST to have inserts go to the first or last table, or a value of NO to prevent inserts. See Section 14.3, “The MERGE Storage Engine”.
MAX_ROWS
The maximum number of rows you plan to store in the table. This is not a hard limit, but rather a hint to the storage engine that the table must be able to store at least this many rows.
MIN_ROWS
The minimum number of rows you plan to store in the table.
PACK_KEYS
PACK_KEYS takes effect only with MyISAM tables. Set this option to 1 if you want to have smaller indexes. This usually makes updates slower and reads faster. Setting the option to 0 disables all packing of keys. Setting it to DEFAULT tells the storage engine to pack only long CHAR or VARCHAR columns.
If you do not use PACK_KEYS, the default is to pack strings, but not numbers. If you use PACK_KEYS=1, numbers are packed as well.
When packing binary number keys, MySQL uses prefix compression:
Every key needs one extra byte to indicate how many bytes of the previous key are the same for the next key.
The pointer to the row is stored in high-byte-first order directly after the key, to improve compression.
This means that if you have many equal keys on two consecutive rows, all following “same” keys usually only take two bytes (including the pointer to the row). Compare this to the ordinary case where the following keys takes storage_size_for_key + pointer_size (where the pointer size is usually 4). Conversely, you get a significant benefit from prefix compression only if you have many numbers that are the same. If all keys are totally different, you use one byte more per key, if the key is not a key that can have NULL values. (In this case, the packed key length is stored in the same byte that is used to mark if a key is NULL.)
PASSWORD
This option is unused. If you have a need to scramble your .frm files and make them unusable to any other MySQL server, please contact our sales department.
ROW_FORMAT
Defines how the rows should be stored. For MyISAM tables, the option value can be FIXED or DYNAMIC for static or variable-length row format. myisampack sets the type to COMPRESSED. See Section 14.1.3, “MyISAM Table Storage Formats”.
Starting with MySQL 5.0.3, for InnoDB tables, rows are stored in compact format (ROW_FORMAT=COMPACT) by default. The non-compact format used in older versions of MySQL can still be requested by specifying ROW_FORMAT=REDUNDANT.
RAID_TYPE
RAID support has been removed as of MySQL 5.0. For information on RAID, see http://dev.mysql.com/doc/refman/4.1/en/create-table.html.
UNION
UNION is used when you want to access a collection of identical MyISAM tables as one. This works only with MERGE tables. See Section 14.3, “The MERGE Storage Engine”.
You must have SELECT, UPDATE, and DELETE privileges for the tables you map to a MERGE table. (Note: Formerly, all tables used had to be in the same database as the MERGE table itself. This restriction no longer applies.)
You can create one table from another by adding a SELECT statement at the end of the CREATE TABLE statement:
CREATE TABLEnew_tblSELECT * FROMorig_tbl;
MySQL creates new columns for all elements in the SELECT. For example:
mysql>CREATE TABLE test (a INT NOT NULL AUTO_INCREMENT,->PRIMARY KEY (a), KEY(b))->ENGINE=MyISAM SELECT b,c FROM test2;
This creates a MyISAM table with three columns, a, b, and c. Notice that the columns from the SELECT statement are appended to the right side of the table, not overlapped onto it. Take the following example:
mysql>SELECT * FROM foo;+---+ | n | +---+ | 1 | +---+ mysql>CREATE TABLE bar (m INT) SELECT n FROM foo;Query OK, 1 row affected (0.02 sec) Records: 1 Duplicates: 0 Warnings: 0 mysql>SELECT * FROM bar;+------+---+ | m | n | +------+---+ | NULL | 1 | +------+---+ 1 row in set (0.00 sec)
For each row in table foo, a row is inserted in bar with the values from foo and default values for the new columns.
In a table resulting from CREATE TABLE ... SELECT, columns named only in the CREATE TABLE part come first. Columns named in both parts or only in the SELECT part come after that. The data type of SELECT columns can be overridden by also specifying the column in the CREATE TABLE part.
If any errors occur while copying the data to the table, it is automatically dropped and not created.
CREATE TABLE ... SELECT does not automatically create any indexes for you. This is done intentionally to make the statement as flexible as possible. If you want to have indexes in the created table, you should specify these before the SELECT statement:
mysql> CREATE TABLE bar (UNIQUE (n)) SELECT n FROM foo; Some conversion of data types might occur. For example, the AUTO_INCREMENT attribute is not preserved, and VARCHAR columns can become CHAR columns.
When creating a table with CREATE ... SELECT, make sure to alias any function calls or expressions in the query. If you do not, the CREATE statement might fail or result in undesirable column names.
CREATE TABLE artists_and_works SELECT artist.name, COUNT(work.artist_id) AS number_of_works FROM artist LEFT JOIN work ON artist.id = work.artist_id GROUP BY artist.id;
You can also explicitly specify the data type for a generated column:
CREATE TABLE foo (a TINYINT NOT NULL) SELECT b+1 AS a FROM bar;
Use LIKE to create an empty table based on the definition of another table, including any column attributes and indexes defined in the original table:
CREATE TABLEnew_tblLIKEorig_tbl;
The copy is created using the same version of the table storage format as the original table.
CREATE TABLE ... LIKE does not preserve any DATA DIRECTORY or INDEX DIRECTORY table options that were specified for the original table, or any foreign key definitions.
You can precede the SELECT by IGNORE or REPLACE to indicate how to handle rows that duplicate unique key values. With IGNORE, new rows that duplicate an existing row on a unique key value are discarded. With REPLACE, new rows replace rows that have the same unique key value. If neither IGNORE nor REPLACE is specified, duplicate unique key values result in an error.
To ensure that the binary log can be used to re-create the original tables, MySQL does not allow concurrent inserts during CREATE TABLE ... SELECT.
In some cases, MySQL silently changes column specifications from those given in a CREATE TABLE or ALTER TABLE statement. These might be changes to a data type, to attributes associated with a data type, or to an index specification.
Some silent column specification changes include modifications to attribute or index specifications:
TIMESTAMP display sizes are discarded.
Also note that TIMESTAMP columns are NOT NULL by default.
Columns that are part of a PRIMARY KEY are made NOT NULL even if not declared that way.
Trailing spaces are automatically deleted from ENUM and SET member values when the table is created.
MySQL maps certain data types used by other SQL database vendors to MySQL types. See Section 11.7, “Using Data Types from Other Database Engines”.
If you include a USING clause to specify an index type that is not legal for a given storage engine, but there is another index type available that the engine can use without affecting query results, the engine uses the available type.
Possible data type changes are given in the following list. These occur only up to the versions listed. After that, an error occurs if a column cannot be created using the specified data type.
Before MySQL 5.0.3, VARCHAR columns with a length less than four are changed to CHAR.
Before MySQL 5.0.3, if any column in a table has a variable length, the entire row becomes variable-length as a result. Therefore, if a table contains any variable-length columns (VARCHAR, TEXT, or BLOB), all CHAR columns longer than three characters are changed to VARCHAR columns. This does not affect how you use the columns in any way; in MySQL, VARCHAR is just a different way to store characters. MySQL performs this conversion because it saves space and makes table operations faster. See Chapter 14, Storage Engines.
Before MySQL 5.0.3, a CHAR or VARCHAR column with a length specification greater than 255 is converted to the smallest TEXT type that can hold values of the given length. For example, VARCHAR(500) is converted to TEXT, and VARCHAR(200000) is converted to MEDIUMTEXT. Similar conversions occur for BINARY and VARBINARY, except that they are converted to a BLOB type.
Note that these conversions result in a change in behavior with regard to treatment of trailing spaces.
As of MySQL 5.0.3, a CHAR or BINARY column with a length specification greater than 255 is not silently converted. Instead, an error occurs. From MySQL 5.0.6 on, silent conversion of VARCHAR and VARBINARY columns with a length specification greater than 65,535 does not occur if strict SQL mode is enabled. Instead, an error occurs.
Before MySQL 5.0.10, for a specification of DECIMAL(, if M,D)M is not larger than D, it is adjusted upward. For example, DECIMAL(10,10) becomes DECIMAL(11,10). As of MySQL 5.0.10, DECIMAL(10,10) is created as specified.
To see whether MySQL used a data type other than the one you specified, issue a DESCRIBE or SHOW CREATE TABLE statement after creating or altering the table.
Certain other data type changes can occur if you compress a table using myisampack. See Section 14.1.3.3, “Compressed Table Characteristics”.
DROP {DATABASE | SCHEMA} [IF EXISTS] db_name DROP DATABASE drops all tables in the database and deletes the database. Be very careful with this statement! To use DROP DATABASE, you need the DROP privilege on the database. DROP SCHEMA is a synonym for DROP DATABASE as of MySQL 5.0.2.
Important: When a database is dropped, user privileges on the database are not automatically dropped. See Section 13.5.1.3, “GRANT Syntax”.
IF EXISTS is used to prevent an error from occurring if the database does not exist.
If you use DROP DATABASE on a symbolically linked database, both the link and the original database are deleted.
DROP DATABASE returns the number of tables that were removed. This corresponds to the number of .frm files removed.
The DROP DATABASE statement removes from the given database directory those files and directories that MySQL itself may create during normal operation:
All files with these extensions:
.BAK | .DAT | .HSH | .MRG |
.MYD | .MYI | .TRG | .TRN |
.db | .frm | .ibd | .ndb |
All subdirectories with names that consist of two hex digits 00-ff. These are subdirectories used for RAID tables. (These directories are not removed as of MySQL 5.0, when support for RAID tables was removed. You should convert any existing RAID tables and remove these directories manually before upgrading to MySQL 5.0. See Section 2.4.16.2, “Upgrading from MySQL 4.1 to 5.0”.)
The db.opt file, if it exists.
If other files or directories remain in the database directory after MySQL removes those just listed, the database directory cannot be removed. In this case, you must remove any remaining files or directories manually and issue the DROP DATABASE statement again.
You can also drop databases with mysqladmin. See Section 8.10, “mysqladmin — Client for Administering a MySQL Server”.
DROP INDEXindex_nameONtbl_name
DROP INDEX drops the index named index_name from the table tbl_name. This statement is mapped to an ALTER TABLE statement to drop the index. See Section 13.1.2, “ALTER TABLE Syntax”.
DROP [TEMPORARY] TABLE [IF EXISTS]
tbl_name [, tbl_name] ...
[RESTRICT | CASCADE] DROP TABLE removes one or more tables. You must have the DROP privilege for each table. All table data and the table definition are removed, so be careful with this statement! If any of the tables named in the argument list do not exist, MySQL returns an error indicating by name which non-existing tables it was unable to drop, but it also drops all of the tables in the list that do exist.
Important: When a table is dropped, user privileges on the table are not automatically dropped. See Section 13.5.1.3, “GRANT Syntax”.
Use IF EXISTS to prevent an error from occurring for tables that do not exist. A NOTE is generated for each non-existent table when using IF EXISTS. See Section 13.5.4.28, “SHOW WARNINGS Syntax”.
RESTRICT and CASCADE are allowed to make porting easier. In MySQL 5.0, they do nothing.
Note: DROP TABLE automatically commits the current active transaction, unless you use the TEMPORARY keyword.
The TEMPORARY keyword has the following effects:
The statement drops only TEMPORARY tables.
The statement does not end an ongoing transaction.
No access rights are checked. (A TEMPORARY table is visible only to the client that created it, so no check is necessary.)
Using TEMPORARY is a good way to ensure that you do not accidentally drop a non-TEMPORARY table.
RENAME TABLEtbl_nameTOnew_tbl_name[,tbl_name2TOnew_tbl_name2] ...
This statement renames one or more tables.
The rename operation is done atomically, which means that no other thread can access any of the tables while the rename is running. For example, if you have an existing table old_table, you can create another table new_table that has the same structure but is empty, and then replace the existing table with the empty one as follows (assuming that backup_table does not already exist):
CREATE TABLE new_table (...); RENAME TABLE old_table TO backup_table, new_table TO old_table;
If the statement renames more than one table, renaming operations are done from left to right. If you want to swap two table names, you can do so like this (assuming that tmp_table does not already exist):
RENAME TABLE old_table TO tmp_table,
new_table TO old_table,
tmp_table TO new_table; As long as two databases are on the same filesystem, you can use RENAME TABLE to move a table from one database to another:
RENAME TABLEcurrent_db.tbl_nameTOother_db.tbl_name;
Beginning with MySQL 5.0.2, if there are any triggers associated with a table which is moved to a different database using RENAME TABLE, then the statement fails with the error Trigger in wrong schema.
As of MySQL 5.0.14, RENAME TABLE also works for views, as long as you do not try to rename a view into a different database.
Any privileges granted specifically for the renamed table or view are not migrated to the new name. They must be changed manually.
When you execute RENAME, you cannot have any locked tables or active transactions. You must also have the ALTER and DROP privileges on the original table, and the CREATE and INSERT privileges on the new table.
If MySQL encounters any errors in a multiple-table rename, it does a reverse rename for all renamed tables to return everything to its original state.