MySQL 5.0 Reference Manual :: 13 SQL Statement Syntax :: 13.1 Data Definition Statements :: 13.1.4 ALTER TABLE Syntax

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13.1.4 ALTER TABLE Syntax

13.1.4.1 ALTER TABLE Examples

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

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_type]
  | ADD [CONSTRAINT [symbol]] PRIMARY KEY
        [index_type] (index_col_name,...) [index_type]
  | ADD [CONSTRAINT [symbol]]
        UNIQUE [INDEX|KEY] [index_name]
        [index_type] (index_col_name,...) [index_type]
  | ADD [FULLTEXT|SPATIAL] [INDEX|KEY] [index_name]
        (index_col_name,...) [index_type]
  | ADD [CONSTRAINT [symbol]]
        FOREIGN KEY [index_name] (index_col_name,...)
        reference_definition
  | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  | CHANGE [COLUMN] old_col_name new_col_name column_definition
        [FIRST|AFTER col_name]
  | 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

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

index_type:
    USING {BTREE | HASH}

table_options:
    table_option [[,] table_option] ...  (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.

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 13.1.10, “CREATE TABLE Syntax”, for more information.

table_options signifies table options of the kind that can be used in the CREATE TABLE statement, such as ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, MAX_ROWS, or ROW_FORMAT. For a list of all table options and a description of each, see Section 13.1.10, “CREATE TABLE Syntax”. However, ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.

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 13.7.5.37, “SHOW WARNINGS Syntax”.

If you use ALTER TABLE to change a column specification but DESCRIBE tbl_name indicates that your column was not changed, it is possible that MySQL ignored your modification for one of the reasons described in Section 13.1.10.4, “Silent Column Specification Changes”.

For information on troubleshooting ALTER TABLE, see Section B.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. 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 copy of the original 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.

If you use 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 13.1.20, “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 index re-creation (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, 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 one 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_options signifies table options of the kind that can be used in the CREATE TABLE statement, such as ENGINE, AUTO_INCREMENT, AVG_ROW_LENGTH, MAX_ROWS, or ROW_FORMAT. For a list of all table options and a description of each, see Section 13.1.10, “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;
```
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”.
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 = value as of MySQL 5.0.3, but if the value is less than the current maximum value in the column, no error occurs and the current sequence value is not changed.
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 13.1.10, “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 11.6, “Data Type Default Values”.
DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.15, “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.
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 13.1.8, “CREATE INDEX Syntax”. Before MySQL 5.0.60, USING can be given only before the index column list. As of 5.0.60, 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 13.1.8, “CREATE INDEX Syntax”.
After an ALTER TABLE statement, it may be necessary to run ANALYZE TABLE to update index cardinality information. See Section 13.7.5.18, “SHOW INDEX Syntax”.
ORDER BY enables you to create the new table with the rows in a specific order. 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.

Note

The table does not remain in the specified order after inserts and deletes.

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 because InnoDB always orders table rows according to the clustered index. The same is true for BDB tables that contain a user-defined PRIMARY KEY.
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.
For MyISAM tables, key updating can be controlled explicitly. Use ALTER TABLE ... DISABLE KEYS to tell MySQL to stop updating nonunique indexes. Then use ALTER TABLE ... ENABLE KEYS to re-create missing indexes. MyISAM 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.
If ALTER TABLE for an InnoDB table results in changes to column values (for example, because a column is truncated), InnoDB's FOREIGN KEY constraint checks do not notice possible violations caused by changing the values.
The FOREIGN KEY and REFERENCES clauses are supported by the InnoDB storage engine, which implements ADD [CONSTRAINT [symbol]] FOREIGN KEY [index_name] (...) REFERENCES ... (...). See Section 14.2.3.4, “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 13.1.10, “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 1.8.2, “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 by InnoDB. InnoDB only accepts REFERENCES clauses defined as part of a separate FOREIGN KEY specification.
InnoDB supports the use of ALTER TABLE to drop foreign keys:
```
ALTER TABLE tbl_name DROP FOREIGN KEY fk_symbol;
```
For more information, see Section 14.2.3.4, “InnoDB and FOREIGN KEY Constraints”.
Adding and dropping a foreign key in separate clauses of a single ALTER TABLE statement may be problematic in some cases and is therefore unsupported. Use separate statements for each operation.
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;
```
The tablespace file must have been created on the server into which it is imported later.
See Section 14.2.1.4, “InnoDB File-Per-Table Tablespaces”.
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 [COLLATE collation_name];
```
The statement also changes the collation of all character columns. If you specify no COLLATE clause to indicate which collation to use, the statement uses default collation for the character set. If this collation is inappropriate for the intended table use (for example, if it would change from a case-sensitive collation to a case-insensitive collation), specify a collation explicitly.
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 3 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 20.6.7.35, “mysql_info()”.

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