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13.1.7 ALTER TABLE Syntax

    [alter_specification [, alter_specification] ...]

  | 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] ...
        [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,...)
  | ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  | CHANGE [COLUMN] old_col_name new_col_name column_definition
        [FIRST|AFTER col_name]
  | [DEFAULT] CHARACTER SET [=] charset_name [COLLATE [=] collation_name]
  | CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
  | DROP [COLUMN] col_name
  | DROP {INDEX|KEY} index_name
  | DROP FOREIGN KEY fk_symbol
  | MODIFY [COLUMN] col_name column_definition
        [FIRST | AFTER col_name]
  | ORDER BY col_name [, col_name] ...
  | RENAME [TO|AS] new_tbl_name
  | ADD PARTITION (partition_definition)
  | DROP PARTITION partition_names
  | TRUNCATE PARTITION {partition_names | ALL}
  | 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}

    col_name [(length)] [ASC | DESC]


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

    table_option [[,] table_option] ...

    AUTO_INCREMENT [=] value
  | AVG_ROW_LENGTH [=] value
  | [DEFAULT] CHARACTER SET [=] charset_name
  | CHECKSUM [=] {0 | 1}
  | [DEFAULT] COLLATE [=] collation_name
  | COMMENT [=] 'string'
  | CONNECTION [=] 'connect_string'
  | {DATA|INDEX} DIRECTORY [=] 'absolute path to directory'
  | DELAY_KEY_WRITE [=] {0 | 1}
  | ENGINE [=] engine_name
  | KEY_BLOCK_SIZE [=] value
  | MAX_ROWS [=] value
  | MIN_ROWS [=] value
  | PACK_KEYS [=] {0 | 1 | DEFAULT}
  | PASSWORD [=] 'string'
  | STATS_SAMPLE_PAGES [=] value
  | UNION [=] (tbl_name[,tbl_name]...)

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.

There are several additional aspects to the ALTER TABLE statement, described under the following topics in this section:

Table Options

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 descriptions of all table options, see Section 13.1.17, “CREATE TABLE Syntax”. However, ALTER TABLE ignores DATA DIRECTORY and INDEX DIRECTORY when given as table options. ALTER TABLE permits them only as partitioning options, and, as of MySQL 5.6.35, requires that you have the FILE privilege.

Use of table options with ALTER TABLE provides a convenient way of altering single table characteristics. For example:

To verify that the table options were changed as intended, use SHOW CREATE TABLE, or query the INFORMATION_SCHEMA.TABLES table.

Performance and Storage Considerations

Some ALTER TABLE operations can be performed in place without making a temporary copy of the table. In-place operations tend to be very fast.

Other ALTER TABLE operations perform the alteration on a temporary copy of the table, which can require more time, particularly for large tables.

In-place ALTER TABLE operations that do not require creating a temporary copy of the original table include:

  • ALTER TABLE operations on InnoDB tables that are supported by the InnoDB online DDL feature. For an overview of supported operations, see Section 14.13.1, “Online DDL Overview”. For information about performance and concurrency of online DDL operations, see Section 14.13.2, “Online DDL Performance, Concurrency, and Space Requirements”.

  • ALTER TABLE tbl_name RENAME TO new_tbl_name. When run without other options, MySQL renames 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”.) 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 are immediate because the server only needs to alter the table .frm file, not touch table contents. The following changes are made in this way:

    • Renaming a column.

    • Changing the default value of a column (except for NDB tables; see Limitations of NDB 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 requires a table copy. Adding members in the middle of the list causes renumbering of existing members, which requires a table copy.

  • Renaming an index, except for InnoDB.

  • Adding or dropping an index, for InnoDB and NDB. See Section 14.13.1, “Online DDL Overview”.

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

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.

ALTER TABLE operations that are not performed in place make 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 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.

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 InnoDB tables, a table-copying ALTER TABLE operation on table that resides in a shared tablespace such as the system tablespace can increase the amount of space used by the tablespace. Such operations require as much additional space as the data in the table plus indexes. For a table that resides in a shared tablespace, the additional space used during a table-copying ALTER TABLE operation is not released back to the operating system as it is for a table that resides in a file-per-table tablespace.

To force use of the table-copy method for an ALTER TABLE operation that would otherwise not use it, set the old_alter_table system variable to ON, or specify 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.

Specifying ALGORITHM=DEFAULT is the same a specifying no ALGORITHM clause at all, in which case ALGORITM=INPLACE is used if supported by the storage engine. Otherwise, ALGORITHM=COPY is used.

An ALTER TABLE operation run with the ALGORITHM=COPY clause prevents concurrent DML operations. Concurrent queries are still allowed. That is, a table-copying operation always includes at least the concurrency restrictions of LOCK=SHARED (allow queries but not DML). You can further restrict concurrency for such operations by specifying LOCK=EXCLUSIVE, which prevents DML and queries.

As of MySQL 5.6.16, ALTER TABLE upgrades MySQL 5.5 temporal columns to 5.6 format for ADD COLUMN, CHANGE COLUMN, MODIFY COLUMN, ADD INDEX, and FORCE operations. This conversion cannot be done using the INPLACE algorithm because the table must be rebuilt, so specifying ALGORITHM=INPLACE in these cases results in an error. Specify ALGORITHM=COPY if necessary.

If an ALTER TABLE operation on a multicolumn index used to partition a table by KEY changes the order of the columns, it can only be performed using ALGORITHM=COPY.

NDB Cluster supports online ALTER TABLE operations using the ALGORITHM=INPLACE syntax in MySQL NDB Cluster 7.3 and later. NDB Cluster also supports an older syntax specific to NDB that uses the ONLINE and OFFLINE keywords. These keywords are deprecated beginning with MySQL NDB Cluster 7.3; they continue to be supported in MySQL NDB Cluster 7.4, but are subject to removal in a future version of NDB Cluster. See Section, “ALTER TABLE Online Operations in NDB Cluster”, for the exact syntax and other particulars.

ALTER TABLE with ADD PARTITION, DROP PARTITION, COALESCE PARTITION, REBUILD PARTITION, or REORGANIZE PARTITION does not create 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.

Locking and Concurrency Control

To control the level of concurrent reading and writing of the table while it is being altered, use the LOCK clause. Specifying a non-default value for this clause enables you to 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:


    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.


    If supported, permit concurrent reads and writes. Otherwise, an error occurs.


    If supported, permit concurrent reads but block writes. Writes are 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, an error occurs.


    Enforce exclusive access. This is done even if concurrent reads/writes are supported by the storage engine for the given ALGORITHM clause (if any) and ALTER TABLE operation.


Pending INSERT DELAYED statements are lost if a table is write locked and ALTER TABLE is used to modify the table structure.

Adding and Dropping Columns

Use ADD to add new columns to a table, and DROP to remove existing columns.

DROP col_name is a MySQL extension to standard SQL.

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.

If a table contains only one column, the column cannot be dropped. If what you intend is to remove the table, use the DROP TABLE statement instead.

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.

Renaming, Redefining, and Reordering Columns

The CHANGE, MODIFY, and ALTER clauses enable the names and definitions of existing columns to be altered. They have these comparative characteristics:


    • Can rename a column and change its definition, or both.

    • Has more capability than MODIFY, but at the expense of convenience for some operations. CHANGE requires naming the column twice if not renaming it.

    • With FIRST or AFTER, can reorder columns.


    • Can change a column definition but not its name.

    • More convenient than CHANGE to change a column definition without renaming it.

    • With FIRST or AFTER, can reorder columns.

  • ALTER: Used only to change a column default value.

CHANGE is a MySQL extension to standard SQL. MODIFY is a MySQL extension for Oracle compatibility.

To alter a column to change both its name and definition, use CHANGE, specifying the old and new names and the new definition. For example, to rename an INT NOT NULL column from a to b and change its definition to use the BIGINT data type while retaining the NOT NULL attribute, do this:


To change a column definition but not its name, use CHANGE or MODIFY. With CHANGE, the syntax requires two column names, so you must specify the same name twice to leave the name unchanged. For example, to change the definition of column b, do this:


MODIFY is more convenient to change the definition without changing the name because it requires the column name only once:


To change a column name but not its definition, use CHANGE. The syntax requires a column definition, so to leave the definition unchanged, you must respecify the definition the column currently has. For example, to rename an INT NOT NULL column from b to a, do this:


For column definition changes using CHANGE or MODIFY, the 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, intending to change only INT to BIGINT:


That statement changes the data type from INT to BIGINT, but it also drops the UNSIGNED, DEFAULT, and COMMENT attributes. To retain them, the statement must include them explicitly:


For data type changes using CHANGE or MODIFY, MySQL tries to convert existing column values to the new type as well as possible.


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 5.1.8, “Server SQL Modes”).

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.

For columns renamed by CHANGE, MySQL automatically renames these references to the renamed column:

  • Indexes that refer to the old column, including indexes and disabled MyISAM indexes.

  • Foreign keys that refer to the old column.

For columns renamed by CHANGE, MySQL does not automatically rename these references to the renamed column:

  • Partition expressions that refer to the renamed column. You must use CHANGE to redefine such expressions in the same ALTER TABLE statement as the one that renames the column.

  • Views and stored programs that refer to the renamed column. You must manually alter the definition of these objects to refer to the new column name.

To reorder columns within a table, use FIRST 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 11.6, “Data Type Default Values”.

Primary Keys and Indexes

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, “Primary Key Optimization”.

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.

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.

As of MySQL 5.6.17, the IGNORE clause is deprecated and its use generates a warning. IGNORE is removed in MySQL 5.7.

DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.24, “DROP INDEX Syntax”. To determine index names, use SHOW INDEX FROM tbl_name.

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. For details about permissible index_option values, see Section 13.1.13, “CREATE INDEX Syntax”.

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.

After an ALTER TABLE statement, it may be necessary to run ANALYZE TABLE to update index cardinality information. See Section, “SHOW INDEX Syntax”.

Foreign Keys

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, “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 1.7.2, “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:


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


Partitioned InnoDB tables do not support foreign keys. This restriction does not apply to NDB tables, including those explicitly partitioned by [LINEAR] KEY. For more information, see Section 19.6.2, “Partitioning Limitations Relating to Storage Engines”.

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;

Adding and dropping a foreign key in the same ALTER TABLE statement is supported for ALTER TABLE ... ALGORITHM=INPLACE but not for ALTER TABLE ... ALGORITHM=COPY.

ALTER TABLE tbl_name RENAME new_tbl_name changes internally generated foreign key constraint names and user-defined foreign key constraint names that contain the string tbl_name_ibfk_ to reflect the new table name. InnoDB interprets foreign key constraint names that contain the string tbl_name_ibfk_ as internally generated names.

Changing the Character Set

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 changes 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 does not fit in a TEXT column's length bytes, so MySQL converts 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:


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 attribute and a subsequent CONVERT TO operation will not apply to them.

If charset_name is DEFAULT, the database character set is used.


The CONVERT TO operation converts column values between the original and named 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:


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:


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).

When the foreign_key_checks system variable is enabled, which is the default setting, character set conversion is not permitted on tables that include a character string column used in a foreign key constraint. The workaround is to disable foreign_key_checks before performing the character set conversion. You must perform the conversion on both tables involved in the foreign key constraint before re-enabling foreign_key_checks. If you re-enable foreign_key_checks after converting only one of the tables, an ON DELETE CASCADE or ON UPDATE CASCADE operation could corrupt data in the referencing table due to implicit conversion that occurs during these operations (Bug #45290, Bug #74816).

Discarding and Importing InnoDB Tablespaces

An InnoDB table created in its own file-per-table tablespace can be discarded and imported using the DISCARD TABLESPACE and IMPORT TABLESPACE options. These options can be used to import a file-per-table tablespace from a backup or to copy a file-per-table tablespace from one database server to another. See Section 14.7.6, “Copying File-Per-Table Tablespaces to Another Instance”.

Row Order for MyISAM Tables

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 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.


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.

When used on a partitioned table, ALTER TABLE ... ORDER BY orders rows within each partition only.

Partitioning Options

partition_options signifies options that can be used with partitioned tables for repartitioning, to add, drop, discard, merge, and split partitions, and to perform 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, EXCHANGE 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 about partition options, see Section 13.1.17, “CREATE TABLE Syntax”, and Section, “ALTER TABLE Partition Operations”. For information about and examples of ALTER TABLE ... EXCHANGE PARTITION statements, see Section 19.3.3, “Exchanging Partitions and Subpartitions with Tables”.

User Comments
  Posted by Tom S on December 18, 2002
IF you want to change a SET or ENUM column you may
not want to use the ALTER TABLE ... MODIFY
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
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.

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
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
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`),

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
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
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
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.:

  Posted by Rachel McConnell on September 7, 2006
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
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

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

use db_old;
alter table tab_name rename db_new.tab_name;
  Posted by John Walker on May 16, 2008
There seem to any number of convoluted methods (not to mention some finger wagging by purists questioning the practice, even here) for altering the sequence of fields in a MySQL table but ALTER does the job as prescribed. It isn't completely self-evident from the description above so here's what worked for me:

ALTER TABLE tablex CHANGE colx colx colxtype AFTER coly.

That is, you're not changing the name of the column but still need to specify 'oldname newname' as 'oldname oldname'
  Posted by Nadav Kavalerchik on March 26, 2009
  Posted by federico cattozzi on April 10, 2009
If you want to change the table's engine for all tables, you can use this code to generate your sql script.

From MyISAM engine to InnoDB engine: set db_name and db_username then copy and paste the follow lines on a Linux/MacOSX shell.

mysql --user=db_username -p --execute="USE information_schema; SELECT CONCAT(\"ALTER TABLE \`\", TABLE_SCHEMA,\"\`.\`\", TABLE_NAME, \"\` TYPE = InnoDB;\") as MySQLCMD from TABLES where TABLE_SCHEMA = \""${DB_NAME}"\";" > ${DB_NAME}-temp.sql;
#delete first line
sed '/MySQLCMD/d' ${DB_NAME}-temp.sql > ${DB_NAME}-innodb.sql;
mysql --user=db_username -p < ${DB_NAME}-innodb.sql;
rm ${DB_NAME}-temp.sql;
rm ${DB_NAME}-innodb.sql;

You can customize the code above for your OS.

I used code from here:,244395,244421#msg-244421

  Posted by John45 G on May 18, 2009
Found some good alter table here:

  Posted by David Friedman on May 7, 2010
It is okay to add two or more columns in the same query where the before and after clauses refer to newly created columns:

  Posted by Bill Vogel on May 27, 2010
If you use "ALTER TABLE mytable AUTO_INCREMENT = N" the MySQL server will create a new table, copy all the data from the old table to the new, delete the old and then rename the new one even though there is no structural change to the table. The server response will show that all the rows have been "affected", like this:
mysql> alter table mytable auto_increment=1000000;
Query OK, 512691 rows affected (1 min 4.55 sec)
Records: 512691 Duplicates: 0 Warnings: 0

There are potential issues that may arise from the table copy, especially if you didn't expect it! I.e. is there a sufficient amount of free disk space for the second copy of the data, etc., etc..

The bottom line for me is to go back to the "old fashioned way" - just insert a dummy row and explicitly set the AUTO_INCREMENT column to N - 1, then immediately delete the dummy row. The next row that is inserted will start at N and go from there.

  Posted by nabeel khan on June 13, 2010
none of the alter code sql queries were working for me in phpmyadmin

however after digging much, finally found something that may help you all.

use this query code:

ALTER TABLE table_name MODIFY column_to_move column_type AFTER column_to_reference

have explained it with example here:
  Posted by Karen Sanasaryan on August 11, 2010
If you need copy big MYISAM tables, just create a new table with same structure (SHOW CREATE TABLE table_to_copy; CREATE TABLE new_table %old table structure%), then copy old table's .frm, .MYD and .MYI files, rename copied files from old_name.* to new_name.*, then run ANALYZE TABLE new_name. No need to restart MySql.
  Posted by Ayden Bissessar on December 6, 2010
If you want to ALTER a table and set the default value to CURRENT_TIMESTAMP for a timestamp column, the listed syntax of "ALTER TABLE foo ALTER COLUMN ts SET DEFAULT CURRENT_TIMESTAMP" will NOT work.

Instead, the syntax below will work. You also need to put the column name twice, I don't know why, it just works.

  Posted by Arthur Liu on February 8, 2011

When adding a new column to a table, and making it a foreign key, if you get

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint fails

please check :

1, The new column must be an index column,
2, All value in the new column must be valid foreign keys.

Here are correct steps:
(Suppose you have author in volumes table, but want to add it to books table.)

ALTER TABLE books ADD COLUMN `author` int(10) unsigned NOT NULL ;
ALTER TABLE books ADD INDEX (author) ;
UPDATE books SET author = ( SELECT author FROM volumes WHERE = books.volume_id ) ;
ALTER TABLE books ADD FOREIGN KEY (author) REFERENCES `users` (`id`) ;

  Posted by Claude Warren on March 22, 2011
It is not possible to change a column name on an InnoDB table if the column participates in a foreign key without first dropping the foreign key. It does not matter if the column is used to reference another table or if it is referenced by another table in the foreign key.
  Posted by Lincoln Rickwood on June 24, 2011
Stored procedure to add a column (sorry about the lack of indentation, the comment box seems to strip leading spaces.)

delimiter //
drop procedure if exists AddTableColumn //
create procedure AddTableColumn
( in schemaName varchar(128) -- If null use name of current schema;
, in tableName varchar(128) -- If null an exception will be thrown.
, in columnName varchar(128) -- If null an exception will be thrown.
, in columnDefinition varchar(1024) -- E.g. 'int not null default 1' (Can include comment here if columnComment is null.)
, in columnComment varchar(1024) -- E.g. 'comment about column.' Can be null. (If null then the comment can be included in columnDefinition.)
, in ifPresent enum('leaveUnchanged', 'dropAndReplace', 'modifyExisting') -- null=leaveUnchanged.
, out outcome tinyint(1) -- 0=unchanged, 1=replaced, 2=modified, 4=added.
declare doDrop tinyint(1) default null;
declare doAdd tinyint(1) default null;
declare doModify tinyint(1) default null;
declare tmpSql varchar(4096) default '';

set schemaName = coalesce(schemaName, schema());
set ifPresent = coalesce(ifPresent, 'leaveUnchanged');
-- select schemaName, ifPresent;
if exists
(select *
from `information_schema`.`COLUMNS`
where `COLUMN_NAME` = columnName
and `TABLE_NAME` = tableName
and `TABLE_SCHEMA` = schemaName
-- select 'exists';
if (ifPresent = 'leaveUnchanged')
set doDrop = 0;
set doAdd = 0;
set doModify = 0;
set outcome = 0;
elseif (ifPresent = 'dropAndReplace')
set doDrop = 1;
set doAdd = 1;
set doModify = 0;
set outcome = 1;
elseif (ifPresent = 'modifyExisting')
set doDrop = 0;
set doAdd = 0;
set doModify = 1;
set outcome = 2;
end if;
-- select 'not-exists';
set doDrop = 0;
set doAdd = 1;
set doModify = 0;
set outcome = 4;
end if;

-- select doDrop, doAdd, doModify, outcome;
if (doDrop = 1)
set tmpSql = concat( 'alter table `', schemaName, '`.`', tableName, '` drop column `', columnName, '` ');
set @sql = tmpSql;
prepare tmp_stmt from @sql;
execute tmp_stmt;
deallocate prepare tmp_stmt;
end if;

if (doAdd = 1)
set tmpSql = concat( 'alter table `', schemaName, '`.`', tableName, '` add column `', columnName, '` ', columnDefinition);
if (columnComment is not null)
set tmpSql = concat(tmpSql, ' comment "', columnComment, '"');
end if;
set @sql = tmpSql;
prepare tmp_stmt from @sql;
execute tmp_stmt;
deallocate prepare tmp_stmt;
end if;

if (doModify = 1)
set tmpSql = concat( 'alter table `', schemaName, '`.`', tableName, '` modify column `', columnName, '` ', columnDefinition);
if (columnComment is not null)
set tmpSql = concat(tmpSql, ' comment "', columnComment, '"');
end if;
set @sql = tmpSql;
prepare tmp_stmt from @sql;
execute tmp_stmt;
deallocate prepare tmp_stmt;
end if;
end; //

  Posted by Pavel Tishkin on September 8, 2011
CREATE PROCEDURE `clone_table`(IN tablex TEXT)
DECLARE sqls,tablexs,cols TEXT;


SET tablexs = CONCAT(tablex,'_tmp');

SET @sql=CONCAT('ALTER TABLE `',tablex,'` RENAME TO `',tablexs,'`');
PREPARE stmt1 FROM @sql;
EXECUTE stmt1;

SET @sql=CONCAT('CREATE TABLE ',tablex,' LIKE ',tablexs);
PREPARE stmt1 FROM @sql;
EXECUTE stmt1;

SELECT column_name INTO cols
FROM `information_schema`.`COLUMNS`
WHERE table_name=tablexs
AND table_schema=DATABASE()
AND column_key='pri' AND extra='auto_increment';

SELECT `auto_increment` INTO @id
FROM `information_schema`.`TABLES`
WHERE table_name=tablexs
AND table_schema=DATABASE();

SET @sql=CONCAT('ALTER TABLE `',tablex,'` AUTO_INCREMENT=',@id);
PREPARE stmt1 FROM @sql;
EXECUTE stmt1;
END $$


CREATE PROCEDURE `clone_table_sync`(IN table_namex VARCHAR(100), IN idx INT)
DECLARE table_namexs TEXT;


SET table_namexs = CONCAT(table_namex,'_tmp');

SELECT `auto_increment` INTO ids
FROM `information_schema`.`TABLES`
WHERE table_name=table_namexs
AND table_schema=DATABASE();

WHILE ids>idx DO
SET @sql=CONCAT('INSERT IGNORE INTO `',table_namex,'` SELECT * FROM `',table_namexs,
'` WHERE i_id>',idx,' AND i_id<=',(idx+5000));
PREPARE stmt1 FROM @sql;
EXECUTE stmt1;

-- SELECT SLEEP(1) INTO @tmp2;
SET idx=idx+5000;

END $$
  Posted by Jonathan Evans on December 3, 2011
An addition to John Walker's example above for reordering the columns in a table.

ALTER TABLE tablex CHANGE colx colx colxtype AFTER coly

Not only do you need to specify 'oldname newname' as 'oldname oldname', you also need to respecify the type of 'oldname' as 'colxtype' (or change it of course) for the statement to work.
  Posted by Ferdous Khan on January 12, 2012
ALTER TABLE sales.order ADD UNIQUE(order_ref);

This command will update 'sales' databases 'order' tables 'order_ref' column to become uniquely indexed. If the column already have some duplicate data, an error message will be prompted.
  Posted by Robert Kline on August 12, 2014
Note that if you include 'UNIQUE' as part of the column definition in an ALTER TABLE MODIFY COLUMN ... statement for a column which was original defined as UNIQUE, MySQL will create a second UNIQUE index. What you need to do if you don't want that second index (any why would you? as it does nothing but slow things down), and you're dealing with scripted changes which you have to give to a separate DBA team who won't accept any instructions which involve manual work (so they won't examine the results of the ALTER TABLE statement to find out the name of the extra index which needs to be dropped), on a server over which you don't have administrative privileges, is to leave out the UNIQUE keyword from the column definition, knowing that MySQL will leave the UNIQUE constraint in place. It's as if MySQL doesn't consider 'UNIQUE' as part of the column definition (even though the syntax rules include it as part of column_definition).
  Posted by Saif Ullah on September 6, 2014
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 my website . I wasted my 4 days looking and searching all around this..):

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 Rick James on April 30, 2017
There is no one-size-fits-all for messed up characters.

First, there are about 6 "best practices" to doing it right in the first place, as enumerated here: .

Second, there are at least 4 visible symptoms for 5 underlying problems, as enumerated here: "Señor" might be turned into
* `Se?or` -- Question marks
* `Señor` -- Mojibake or Double-encoding
* `Se�or` -- Black diamond
* `Se` -- Truncation

Third, there are 5 possible ways to fix the data. See .
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