MySQL 5.6 Reference Manual :: 13 SQL Statement Syntax :: 13.1 Data Definition Statements :: 13.1.7 ALTER TABLE Syntax

13.1.7.1 ALTER TABLE Partition Operations »

Section Navigation [Toggle]

13.1.7. `ALTER TABLE` Syntax

13.1.7.1. ALTER TABLE Partition Operations
13.1.7.2. ALTER TABLE Online Operations in MySQL Cluster
13.1.7.3. ALTER TABLE Examples

ALTER [IGNORE] TABLE tbl_name
    [alter_specification [, alter_specification] ...]
    [partition_options]

alter_specification:
    table_options
  | ADD [COLUMN] col_name column_definition
        [FIRST | AFTER col_name ]
  | ADD [COLUMN] (col_name column_definition,...)
  | ADD {INDEX|KEY} [index_name]
        [index_type] (index_col_name,...) [index_option] ...
  | ADD [CONSTRAINT [symbol]] PRIMARY KEY
        [index_type] (index_col_name,...) [index_option] ...
  | ADD [CONSTRAINT [symbol]]
        UNIQUE [INDEX|KEY] [index_name]
        [index_type] (index_col_name,...) [index_option] ...
  | ADD FULLTEXT [INDEX|KEY] [index_name]
        (index_col_name,...) [index_option] ...
  | ADD SPATIAL [INDEX|KEY] [index_name]
        (index_col_name,...) [index_option] ...
  | ADD [CONSTRAINT [symbol]]
        FOREIGN KEY [index_name] (index_col_name,...)
        reference_definition
  | ALGORITHM [=] {DEFAULT|INPLACE|COPY}
  | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  | CHANGE [COLUMN] old_col_name new_col_name column_definition
        [FIRST|AFTER col_name]
  | LOCK [=] {DEFAULT|NONE|SHARED|EXCLUSIVE}
  | MODIFY [COLUMN] col_name column_definition
        [FIRST | AFTER col_name]
  | DROP [COLUMN] col_name
  | DROP PRIMARY KEY
  | DROP {INDEX|KEY} index_name
  | DROP FOREIGN KEY fk_symbol
  | DISABLE KEYS
  | ENABLE KEYS
  | RENAME [TO|AS] new_tbl_name
  | ORDER BY col_name [, col_name] ...
  | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
  | [DEFAULT] CHARACTER SET [=] charset_name [COLLATE [=] collation_name]
  | DISCARD TABLESPACE
  | IMPORT TABLESPACE
  | FORCE
  | ADD PARTITION (partition_definition)
  | DROP PARTITION partition_names
  | TRUNCATE PARTITION {partition_names | ALL}
  | COALESCE PARTITION number
  | REORGANIZE PARTITION partition_names INTO (partition_definitions)
  | EXCHANGE PARTITION partition_name WITH TABLE tbl_name
  | ANALYZE PARTITION {partition_names | ALL}
  | CHECK PARTITION {partition_names | ALL}
  | OPTIMIZE PARTITION {partition_names | ALL}
  | REBUILD PARTITION {partition_names | ALL}
  | REPAIR PARTITION {partition_names | ALL}
  | REMOVE PARTITIONING

index_col_name:
    col_name [(length)] [ASC | DESC]

index_type:
    USING {BTREE | HASH}

index_option:
    KEY_BLOCK_SIZE [=] value
  | index_type
  | WITH PARSER parser_name
  | COMMENT 'string'

table_options:
    table_option [[,] table_option] ...  (see CREATE TABLE options)

partition_options:
    (see CREATE TABLE options)

ALTER TABLE changes the structure of a 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 characteristics such as the storage engine used for the table or the table comment.

Partitioning-related clauses for ALTER TABLE can be used with partitioned tables for repartitioning, for adding, dropping, merging, and splitting partitions, and for performing partitioning maintenance. It is possible for an ALTER TABLE statement to contain a PARTITION BY or REMOVE PARTITIONING clause in an addition to other alter specifications, but the PARTITION BY or REMOVE PARTITIONING clause must be specified last after any other specifications. The ADD PARTITION, DROP PARTITION, COALESCE PARTITION, REORGANIZE PARTITION, ANALYZE PARTITION, CHECK PARTITION, and REPAIR PARTITION options cannot be combined with other alter specifications in a single ALTER TABLE, since the options just listed act on individual partitions. For more information, see Section 13.1.7.1, “ALTER TABLE Partition Operations”.

Following the table name, specify the alterations to be made. If none are given, ALTER TABLE does nothing.

The syntax for many of the permissible alterations is similar to clauses of the CREATE TABLE statement. See Section 13.1.17, “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 13.7.5.41, “SHOW WARNINGS Syntax”.

For information on troubleshooting ALTER TABLE, see Section C.5.7.1, “Problems with ALTER TABLE”.

Storage, Performance, and Concurrency Considerations

In most cases, ALTER TABLE makes a temporary copy of the original table. MySQL waits for other operations that are modifying the table, then proceeds. It incorporates the alteration into the copy, deletes the original table, and renames the new one. While ALTER TABLE is executing, the original table is readable by other sessions (with the exception noted shortly). Updates and writes to the table that begin after the ALTER TABLE operation begins are stalled until the new table is ready, then are automatically redirected to the new table without any failed updates. The temporary table is created in the database directory of the new table. This can differ from the database directory of the original table for ALTER TABLE operations that rename the table to a different database.

The exception referred to earlier is that ALTER TABLE blocks reads (not just writes) at the point where it is ready to install a new version of the table .frm file, discard the old file, and clear outdated table structures from the table and table definition caches. At this point, it must acquire an exclusive lock. To do so, it waits for current readers to finish, and blocks new reads (and writes).

For MyISAM tables, you can speed up index re-creation (the slowest part of the alteration process) by setting the myisam_sort_buffer_size system variable to a high value.

For some operations, an in-place ALTER TABLE is possible that does not require a temporary table:

For ALTER TABLE tbl_name RENAME TO new_tbl_name without any other options, MySQL simply renames any files that correspond to the table tbl_name without making a copy. (You can also use the RENAME TABLE statement to rename tables. See Section 13.1.32, “RENAME TABLE Syntax”.) Any privileges granted specifically for the renamed table are not migrated to the new name. They must be changed manually.
Alterations that modify only table metadata and not table data can be made immediately by altering the table's .frm file and not touching table contents. The following changes are fast alterations that can be made this way:
- Renaming a column, except for the InnoDB storage engine before MySQL 5.6.6.
- Changing the default value of a column (except for NDB tables; see Limitations of MySQL Cluster online operations).
- Changing the definition of an ENUM or SET column by adding new enumeration or set members to the end of the list of valid member values, as long as the storage size of the data type does not change. For example, adding a member to a SET column that has 8 members changes the required storage per value from 1 byte to 2 bytes; this will require a table copy. Adding members in the middle of the list causes renumbering of existing members, which requires a table copy.
ALTER TABLE with ADD PARTITION, DROP PARTITION, COALESCE PARTITION, REBUILD PARTITION, or REORGANIZE PARTITION does not create any temporary tables (except when used with NDB tables); however, these operations can and do create temporary partition files.
ADD or DROP operations for RANGE or LIST partitions are immediate operations or nearly so. ADD or COALESCE operations for HASH or KEY partitions copy data between all partitions, unless LINEAR HASH or LINEAR KEY was used; this is effectively the same as creating a new table, although the ADD or COALESCE operation is performed partition by partition. REORGANIZE operations copy only changed partitions and do not touch unchanged ones.
Renaming an index, except for InnoDB.
Adding or dropping an index, for InnoDB and NDB.

You can force an ALTER TABLE operation that would otherwise not require a table copy to use the temporary table method (as supported in MySQL 5.0) by setting the old_alter_table system variable to ON, or specifying ALGORITHM=COPY as one of the alter_specification clauses. If there is a conflict between the old_alter_table setting and an ALGORITHM clause with a value other than DEFAULT, the ALGORITHM clause takes precedence. (ALGORITHM = DEFAULT is the same a specifying no ALGORITHM clause at all.)

Specifying ALGORITHM=INPLACE makes the operation use the in-place technique for clauses and storage engines that support it, and fail with an error otherwise, thus avoiding a lengthy table copy if you try altering a table that uses a different storage engine than you expect. See Section 14.2.12, “InnoDB and Online DDL” for information about online DDL for InnoDB tables.

MySQL Cluster also supports online ALTER TABLE operations using an ONLINE keyword supported only by MySQL Cluster; see Section 13.1.7.2, “ALTER TABLE Online Operations in MySQL Cluster”, for the exact syntax and other particulars.

You can control the level of concurrent reading and writing of the table while it is being altered, using the LOCK clause. Specifying a non-default value for this clause lets you require a certain amount of concurrent access or exclusivity during the alter operation, and halts the operation if the requested degree of locking is not available. The parameters for the LOCK clause are:

```
LOCK = DEFAULT
```
Maximum level of concurrency for the given ALGORITHM clause (if any) and ALTER TABLE operation: Permit concurrent reads and writes if supported. If not, permit concurrent reads if supported. If not, enforce exclusive access.
```
LOCK = NONE
```
If supported, permit concurrent reads and writes. Otherwise, return an error message.
```
LOCK = SHARED
```
If supported, permit concurrent reads but block writes. Note that writes will be blocked even if concurrent writes are supported by the storage engine for the given ALGORITHM clause (if any) and ALTER TABLE operation. If concurrent reads are not supported, return an error message.
```
LOCK = EXCLUSIVE
```
Enforce exclusive access. This will be done even if concurrent reads/writes are supported by the storage engine for the given ALGORITHM clause (if any) and ALTER TABLE operation.

As of MySQL 5.6.3, you can also use ALTER TABLE tbl_name FORCE to perform a “null” alter operation that rebuilds the table. Previously the FORCE option was recognized but ignored.

For NDB tables, operations that add and drop indexes on variable-width columns occur online, without any table copying and without blocking concurrent DML actions for most of their duration. See Section 13.1.7.2, “ALTER TABLE Online Operations in MySQL Cluster”.

Usage Notes

To use ALTER TABLE, you need ALTER, CREATE, and INSERT privileges for the table. Renaming a table requires ALTER and DROP on the old table, ALTER, CREATE, and INSERT on the new 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.
Pending INSERT DELAYED statements are lost if a table is write locked and ALTER TABLE is used to modify the table structure.
table_option signifies a table option of the kind that can be used in the CREATE TABLE statement, such as ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, or MAX_ROWS. For a list of all table options and descriptions of each, see Section 13.1.17, “CREATE TABLE Syntax”. 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;
```
See Section 14.2.7.4, “Converting Tables from MyISAM to InnoDB” for considerations when switching tables to the InnoDB storage engine.
When you specify an ENGINE clause, ALTER TABLE rebuilds the table. This is true even if the table already has the specified storage engine.
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 5.1.7, “Server SQL Modes”.
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 to the value that is currently in use. For both InnoDB and 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 AUTO_INCREMENT column value plus one.
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 permits 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_name, DROP col_name, and DROP INDEX are MySQL extensions to standard SQL.
The word COLUMN is optional and can be omitted.
column_definition clauses use the same syntax for ADD and CHANGE as for CREATE TABLE. See Section 13.1.17, “CREATE TABLE Syntax”.
You can rename a column using a CHANGE old_col_name new_col_name column_definition clause. To do so, specify the old and new column names and the definition that the column currently has. For example, to rename an INTEGER column from a to b, you can do this:
```
ALTER TABLE t1 CHANGE a b INTEGER;
```
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;
```
MODIFY is an extension to ALTER TABLE for Oracle compatibility.
When you use CHANGE or MODIFY, column_definition must include the data type and all attributes that should apply to the new column, other than index attributes such as PRIMARY KEY or UNIQUE. Attributes present in the original definition but not specified for the new definition are not carried forward. Suppose that a column col1 is defined as INT UNSIGNED DEFAULT 1 COMMENT 'my column' and you modify the column as follows:
```
ALTER TABLE t1 MODIFY col1 BIGINT;
```
The resulting column will be defined as BIGINT, but will not include the attributes UNSIGNED DEFAULT 1 COMMENT 'my column'. To retain them, the statement should be:
```
ALTER TABLE t1 MODIFY col1 BIGINT UNSIGNED DEFAULT 1 COMMENT 'my column';
```
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.

Warning

This conversion may result in alteration of data. For example, if you shorten a string column, values may be truncated. To prevent the operation from succeeding if conversions to the new data type would result in loss of data, enable strict SQL mode before using ALTER TABLE (see Section 5.1.7, “Server SQL Modes”).
To add a column at a specific position within a table row, use FIRST or AFTER col_name. The default is to add the column last. You can also use FIRST and AFTER in CHANGE or MODIFY operations to reorder columns within a table.
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 11.5, “Data Type Default Values”.
DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.24, “DROP INDEX Syntax”. If you are unsure of the index name, use SHOW INDEX FROM tbl_name.
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 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.
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 key. If there is no primary key, an error occurs. For information about the performance characteristics of primary keys, especially for InnoDB tables, see Section 8.3.2, “Using Primary Keys”.
If you add a UNIQUE INDEX or PRIMARY KEY to a table, MySQL stores it before any nonunique index to permit detection of duplicate keys as early as possible.
Some storage engines permit you to specify an index type when creating an index. The syntax for the index_type specifier is USING type_name. For details about USING, see Section 13.1.13, “CREATE INDEX Syntax”. The preferred position is after the column list. Support for use of the option before the column list will be removed in a future MySQL release.
index_option values specify additional options for an index. USING is one such option. For details about permissible index_option values, see Section 13.1.13, “CREATE INDEX Syntax”.
After an ALTER TABLE statement, it may be necessary to run ANALYZE TABLE to update index cardinality information. See Section 13.7.5.23, “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 permits 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 permitted as sort criteria; arbitrary expressions are not permitted. This clause should be given last after any other clauses.
ORDER BY does not make sense for InnoDB tables that contain a user-defined clustered index (PRIMARY KEY or NOT NULL UNIQUE index). InnoDB always orders table rows according to such an index if one is present.

Note

When used on a partitioned table, ALTER TABLE ... ORDER BY orders rows within each partition only.
If you use ALTER TABLE on a MyISAM table, all nonunique 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 for a MyISAM table. ALTER TABLE ... DISABLE KEYS tells MySQL to stop updating nonunique indexes. 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 nonunique indexes are disabled, they are ignored for statements such as SELECT and EXPLAIN that otherwise would use them.
Before MySQL 5.6.7, using ALTER TABLE to change the definition of a foreign key column could cause a loss of referential integrity. For example, changing a foreign key column that contained NULL values to be NOT NULL caused the NULL values to be the empty string. Similarly, an ALTER TABLE IGNORE that removed rows in a parent table could break referential integrity.
As of 5.6.7, the server prohibits changes to foreign key columns that have the potential to cause loss of referential integrity. It also prohibits changes to the data type of such columns that may be unsafe. For example, changing VARCHAR(20) to VARCHAR(30) is permitted, but changing it to VARCHAR(1024) is not because that alters the number of length bytes required to store individual values. A workaround is to use ALTER TABLE ... DROP FOREIGN KEY before changing the column definition and ALTER TABLE ... ADD FOREIGN KEY afterward.
The FOREIGN KEY and REFERENCES clauses are supported by the InnoDB and NDB storage engines, which implement ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (...) REFERENCES ... (...). See Section 14.2.7.6, “InnoDB and 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 13.1.17, “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 1.8.5, “MySQL Differences from Standard SQL”.
For ALTER TABLE, unlike CREATE TABLE, ADD FOREIGN KEY ignores index_name if given and uses an automatically generated foreign key name. As a workaround, include the CONSTRAINT clause to specify the foreign key name:
```
ADD CONSTRAINT name FOREIGN KEY (....) ...
```
Important

The inline REFERENCES specifications where the references are defined as part of the column specification are silently ignored. MySQL only accepts REFERENCES clauses defined as part of a separate FOREIGN KEY specification.

Note

Partitioned InnoDB tables do not support foreign keys. This restriction does not apply to NDB tables, including those explicitly partitioned by [LINEAR] KEY. See Section 18.6.2, “Partitioning Limitations Relating to Storage Engines”, for more information.
The InnoDB and NDB storage engines support the use of ALTER TABLE to drop foreign keys:
```
ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;
```
For more information, see Section 14.2.7.6, “InnoDB and FOREIGN KEY Constraints”.
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 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 modify the table contents while the tablespace file is discarded results in an error. You can perform the DDL operations listed in Section 14.2.12, “InnoDB and Online DDL” while the tablespace file is discarded.
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;
```
The tablespace file need not necessarily have been created on the server into which it is imported later. In MySQL 5.6, importing a tablespace file from another server works if the both servers have GA (General Availablility) status and their versions are within the same series. Otherwise, the file must have been created on the server into which it is imported.

Note

The ALTER TABLE ... IMPORT TABLESPACE feature does not enforce foreign key constraints on imported data.

See Section 14.2.6.2, “InnoDB File-Per-Table Mode”.
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 TABLE tbl_name CONVERT TO CHARACTER SET charset_name;
```
For a column that has a data type of VARCHAR or one of the TEXT types, CONVERT TO CHARACTER SET will change the data type as necessary to ensure that the new column is long enough to store as many characters as the original column. For example, a TEXT column has two length bytes, which store the byte-length of values in the column, up to a maximum of 65,535. For a latin1 TEXT column, each character requires a single byte, so the column can store up to 65,535 characters. If the column is converted to utf8, each character might require up to three bytes, for a maximum possible length of 3 × 65,535 = 196,605 bytes. That length will not fit in a TEXT column's length bytes, so MySQL will convert the data type to MEDIUMTEXT, which is the smallest string type for which the length bytes can record a value of 196,605. Similarly, a VARCHAR column might be converted to MEDIUMTEXT.
To avoid data type changes of the type just described, do not use CONVERT TO CHARACTER SET. Instead, use MODIFY to change individual columns. For example:
```
ALTER TABLE t MODIFY latin1_text_col TEXT CHARACTER SET utf8;
ALTER TABLE t MODIFY latin1_varchar_col VARCHAR(M) CHARACTER SET utf8;
```
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.
If charset_name is DEFAULT, the database character set is used.
Warning

The CONVERT TO 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.
To change only the default character set for a table, use this statement:
```
ALTER TABLE tbl_name DEFAULT CHARACTER SET charset_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 columns that you add to a table later (for example, with ALTER TABLE ... ADD column).

With the mysql_info() C API function, you can find out how many rows were copied by ALTER TABLE, and (when IGNORE is used) how many rows were deleted due to duplication of unique key values. See Section 22.8.7.35, “mysql_info()”.

Previous / Next / Up / Table of Contents

User Comments

Posted by Tom S on December 18 2002 3:27pm

13.1.7. ALTER TABLE Syntax

Storage, Performance, and Concurrency Considerations

Usage Notes

User Comments

13.1.7. `ALTER TABLE` Syntax