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MySQL 5.0 Reference Manual :: 12 SQL Statement Syntax :: 12.1 Data Definition Statements :: 12.1.2 ALTER TABLE Syntax

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12.1.2. `ALTER TABLE` Syntax

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

alter_specification:
    table_option ...
  | 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] 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 | RTREE}

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 12.1.5, “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 12.5.4.28, “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 12.1.5.1, “Silent Column Specification Changes”.

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. The temporary table is created in the database directory of the new table. This can be different from the database directory of the original table if ALTER TABLE is renaming 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. (You can also use the RENAME TABLE statement to rename tables. See Section 12.1.9, “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.

For information on troubleshooting ALTER TABLE, see Section B.1.7.1, “Problems with ALTER TABLE”.

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, such as ENGINE, AUTO_INCREMENT, or AVG_ROW_LENGTH. (Section 12.1.5, “CREATE TABLE Syntax”, lists all table options.) 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 5.1.6, “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 = 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 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_name, DROP col_name, and DROP 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. See Section 12.1.5, “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;
```
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.

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 5.1.6, “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 10.1.4, “Data Type Default Values”.
DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 12.1.7, “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 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, 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 type_name. For details about USING, see Section 12.1.4, “CREATE INDEX Syntax”.
After an ALTER TABLE statement, it may be necessary to run ANALYZE TABLE to update index cardinality information. See Section 12.5.4.13, “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.

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. The same is true for BDB tables that contain a user-defined PRIMARY KEY.
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 for a MyISAM table. ALTER TABLE ... DISABLE KEYS tells MySQL to stop updating non-unique 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 non-unique 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 (...) REFERENCES ... (...). See Section 13.2.6.4, “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 12.1.5, “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 1.8.5, “MySQL Differences from Standard SQL”.

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 13.2.6.4, “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 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;
```
See Section 13.2.3.1, “Using Per-Table Tablespaces”.
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 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 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, and (when IGNORE is used) how many rows were deleted due to duplication of unique key values. See Section 24.2.3.35, “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 an index on column d and a UNIQUE index on column a:

ALTER TABLE t2 ADD INDEX (d), ADD UNIQUE (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 automatically. For MyISAM tables, you can set the first sequence number by executing SET INSERT_ID=value before ALTER TABLE or by using the AUTO_INCREMENT=value table option. See Section 12.5.3, “SET 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;

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User Comments

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

[Delete] [Edit]

IF you want to change a SET or ENUM column you may
not want to use the ALTER TABLE ... MODIFY
syntax.
It tries to keep the actual string values and not
the integer representation of the values, even
though they are stored as integers.
For example, if you just want to make a change in
spelling of the values in your enum column or your
set column, consider doing it like this:
ALTER TABLE table ADD new_column ...;
UPDATE table SET new_column = old_column + 0;
ALTER TABLE table DROP old_column;

Posted by David Bell on August 15 2003 1:08am

[Delete] [Edit]

You can use Alter Table to optimise a table without locking out selects (only writes), by altering a column to be the same as it's current definition. This is better than using repair table which obtains a read/write lock.

E.g.
mysql> describe Temp_Table;

+-------+------------------+------+-----+---------+-------+
| Field | Type             | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+-------+
| ID    | int(10) unsigned | YES  |     | NULL    |       |
+-------+------------------+------+-----+---------+-------+

1 row in set (0.00 sec)

mysql> alter table Temp_Table change column ID ID int unsigned;

This will cause mysql to re-create the table and thus remove any deleted space.

This is useful for 24/7 databases where you don't want to completely lock a table.

Posted by Mark Mackay on July 9 2004 6:11am

[Delete] [Edit]

If you are just changing a column name on a MyISAM table and want to avoid duplicating the entire table, try the following (no warranty provided but worked for me):

For peace-of-mind -- try this with some dummy data first!

1. Backup the <original_table>.frm file from your master table (and the data if you can, but you're probably reading this because you can't).

2. create table with the identical schema to the one you want to alter (type "show create table <tablename> and just change the name to something). Lets say you called the table "rename_temp1"

3. execute the "alter table <rename_temp1> change <old_column_name> <new_column_name> char(128) not null" [substituting your the old definition -- ensuring you keep column type the same]

3. Ensuring you a have made a copy of your original .frm file -- copy the <rename_temp1>.frm file to <original_table>.frm.

4. voila -- all going well your column should be renamed without a full copy in/out (very useful for 140G tables...)

5. probably best to run a myisamchck on the table before making live again

Posted by Hadi Rastgou on July 13 2005 7:14am

[Delete] [Edit]

When you want to drop a UNIQUE KEY in an InnoDb table, have to pay attention not to occure this situation:
Please check that columns used in the UNIQUE KEY are not used as FOREIGN KEY (each of them).
If so, must to drop that Forign keys first.
See Example below please.

UNIQUE KEY `unique` (`id1`, `id2`),
CONSTRAINT `fk_1` FOREIGN KEY (`id1`) REFERENCES `tbl1` (`id`) ON DELETE CASCADE,
CONSTRAINT `fk_2` FOREIGN KEY (`id2`) REFERENCES `tbl2` (`id`) ON DELETE CASCADE

In this situation, you have to drop both FOREIGN KEYs first, in order to can drop the UNIQUE KEY.

Posted by Flemming Funch on November 5 2005 11:44pm

[Delete] [Edit]

If you're trying to convert a whole database to a different character set, and you thought you might have to change the fields one by one, this kind of command is really handy:

ALTER TABLE tablename CONVERT TO CHARACTER SET utf8 COLLATE utf8_general_ci;

However, after using it on a lot of tables I made the grim discovery that for older myisam tables that didn't have any character set, it mangled the length of most varchar fields. Specifically, it divided their length with 3. Bizarrely, it didn't lose the existing data, even though it was longer than the field lengths, but it was a problem with new records and with indexes.

So, if you're going to do a character set converstion, make sure the table already has a character set. That it doesn't might not be immediately obvious, as tools like phpMyAdmin will show the default character set, if the table or the field doesn't have one set.

Posted by Daniel Cottrell on November 21 2005 4:50pm

[Delete] [Edit]

TO ADD A FOREIGN KEY TO AN EXISTING TABLE (I couldn't see a good example) you can do this:

alter table users add foreign key(favGenre) references products_genre(gid);

Where favgenre is the column of the table that has the foreign key and products_genre(gid) is the table and primary key you are referencing.

Posted by Duane Hitz on March 31 2006 8:19am

[Delete] [Edit]

Attempting to "ALTER TABLE ... DROP PRIMARY KEY" on a table when an AUTO_INCREMENT column exists in the key generates an error:

ERROR 1075 (42000): Incorrect table definition; there can be only one auto column and it must be defined as a key.

To make this work without erroring, drop and re-add the new primary key in a single statement, e.g.:

ALTER TABLE mytable DROP PRIMARY KEY, ADD PRIMARY KEY(col1,col2);

Posted by Rachel McConnell on September 7 2006 11:15pm

[Delete] [Edit]

You can't drop a NOT NULL constraint on a column the way you can a foreign key or an index, or a default. Instead, just use the 'change' or 'modify' syntax and leave off the NOT NULL bit:

alter table table_name modify col_name bigint default null;

Any pre-existing indexes or foreign keys on the column are not affected.

Posted by Geoffrey Hoffman on June 17 2007 8:58pm

[Delete] [Edit]

If you are trying to change the case of a table name using the ALTER TABLE syntax and it appears to fail silently,

or if you try to RENAME TABLE something TO soMeThiNg and get a 'table already exists' error,

or if you try to CREATE TABLE MixedCaseTableName and get a table named mixedcasetablename, these are not bugs:

See: Identifier Case Sensitivity
http://dev.mysql.com/doc/refman/5.0/en/identifier-case-sensitivity.html

If your development environment has MySQL5 and you're hosting on MySQL4 you can get 'table not found' errors based on the case of the table names.

Posted by Cengiz Gunay on November 27 2007 11:20pm

[Delete] [Edit]

As mentioned above, ALTER TABLE is going to make a temporary copy of the whole table data (.MYD) in the same directory as the original table and not in the temporary directories given to MySQL.

In some cases a third copy of the table (.TMD) is made. This means you must have up to three copies of free space in that directory. Unfortunately MySQL does not break the files into pieces if it runs out of space.

As a table grows larger this process becomes more expensive. Therefore, keys and indices must be added as early as possible to large tables in spite of the update cost that comes with each insert.

Posted by Reinhard Vielhaber on March 14 2008 10:33am

[Delete] [Edit]

For moving a table from one database to another just do:

use db_old;
alter table tab_name rename db_new.tab_name;

Add your own comment.

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12.1.2. ALTER TABLE Syntax

Warning

Important

Warning

Important

User Comments

12.1.2. `ALTER TABLE` Syntax