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, then for rows with duplicates on a unique key, only the first row is used. The others conflicting rows are deleted. Wrong values are truncated to the closest matching acceptable value.

You can issue multiple ADD, ALTER, DROP, and CHANGE clauses in a single ALTER TABLE statement, separated by commas. This is a MySQL extension to standard SQL, which allows only one of each clause per ALTER TABLE statement. For example, to drop multiple columns in a single statement:

mysql> 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 purely optional and can be omitted.

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. There is no need to create a temporary table. (You can also use the RENAME TABLE statement to rename tables. See Section 13.1.9, “RENAME TABLE Syntax”.)

column_definition clauses use the same syntax for ADD and CHANGE as for CREATE TABLE. Note that this syntax includes the column name, not just the column type. See Section 13.1.5, “CREATE TABLE Syntax”.

You can rename a column using a CHANGE old_col_name column_definition clause. To do so, specify the old and new column names and the type that the column currently has. For example, to rename an INTEGER column from a to b, you can do this:

mysql> 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:

mysql> ALTER TABLE t1 CHANGE b b BIGINT NOT NULL;

However, as of MySQL 3.22.16a, you can also use MODIFY to change a column's type without renaming it:

mysql> 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 column type using CHANGE or MODIFY, MySQL tries to convert existing column values to the new type as well as possible.

In MySQL 3.22 or later, you can use FIRST or AFTER col_name to add a column at a specific position within a table row. The default is to add the column last. From MySQL 4.0.1 on, you can also use FIRST and AFTER in CHANGE or MODIFY operations.

ALTER COLUMN specifies a new default value for a column or removes the old default value. 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 13.1.5, “CREATE TABLE Syntax”.

DROP INDEX removes an index. This is a MySQL extension to standard SQL. See Section 13.1.7, “DROP INDEX Syntax”.

If columns are dropped from a table, the columns are also removed from any index of which they are a part. If all columns that make up an index are dropped, the index is dropped as well.

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

DROP PRIMARY KEY drops the primary index. (Prior to MySQL 4.1.2, if no primary index exists, DROP PRIMARY KEY drops the first UNIQUE index in the table. MySQL marks the first UNIQUE key as the PRIMARY KEY if no PRIMARY KEY was specified explicitly.)

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.

ORDER BY allows 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 mainly useful when you know that you are mostly going to query the rows in a certain order; by using this option after big 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.

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.

As of MySQL 4.0, this feature can be activated explicitly. ALTER TABLE ... DISABLE KEYS tells MySQL to stop updating non-unique indexes for a MyISAM table. ALTER TABLE ... ENABLE KEYS then should be used to re-create missing indexes. MySQL does this with a special algorithm that is much faster than inserting keys one by one, so disabling keys before performing bulk insert operations should give a considerable speedup. Using ALTER TABLE ... DISABLE KEYS requires the INDEX privilege in addition to the privileges mentioned earlier.

The FOREIGN KEY and REFERENCES clauses are supported by the InnoDB storage engine, which implements ADD [CONSTRAINT [symbol]] FOREIGN KEY (...) REFERENCES ... (...). See Section 15.7.4, “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 13.1.5, “CREATE TABLE Syntax”. The reason for accepting but ignoring syntax clauses is for compatibility, to make it easier to port code from other SQL servers, and to run applications that create tables with references. See Section 1.8.5, “MySQL Differences from Standard SQL”.

You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. You must use separate statements.

Starting from MySQL 4.0.13, InnoDB supports the use of ALTER TABLE to drop foreign keys:

ALTER TABLE yourtablename DROP FOREIGN KEY fk_symbol;

You cannot add a foreign key and drop a foreign key in separate clauses of a single ALTER TABLE statement. You must use separate statements.

For more information, see Section 15.7.4, “FOREIGN KEY Constraints”.

ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.

From MySQL 4.1.2 on, 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;

This is useful, for example, after upgrading from MySQL 4.0.x to 4.1.x. See Section 10.10, “Upgrading Character Sets from MySQL 4.0”.

Warning: The preceding operation converts column values between the character sets. This is not what you want if you have a column in one character set (like latin1) but the stored values actually use some other, incompatible character set (like utf8). In this case, you have to do the following for each such column:

ALTER TABLE t1 CHANGE c1 c1 BLOB;
ALTER TABLE t1 CHANGE c1 c1 TEXT CHARACTER SET utf8;

The reason this works is that there is no conversion when you convert to or from BLOB columns.

If you specify CONVERT TO CHARACTER SET binary, the CHAR, VARCHAR, and TEXT columns are converted to their corresponding binary string types (BINARY, VARBINARY, BLOB). This means that the columns no longer will have a character set and a subsequent CONVERT TO operation will not apply to them.

To change only the default character set for a table, use this statement:

ALTER 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 a new column which you add to a table (for example, with ALTER TABLE ... ADD column).

Warning: From MySQL 4.1.2 and up, ALTER TABLE ... DEFAULT CHARACTER SET and ALTER TABLE ... CHARACTER SET are equivalent and change only the default table character set. In MySQL 4.1 releases before 4.1.2, ALTER TABLE ... DEFAULT CHARACTER SET changes the default character set, but ALTER TABLE ... CHARACTER SET (without DEFAULT) changes the default character set and also converts all columns to the new character set.

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, then issue this statement:

ALTER TABLE tbl_name IMPORT TABLESPACE;

See Section 15.7.6, “Using Per-Table Tablespaces”.

With the mysql_info() C API function, you can find out how many records were copied, and (when IGNORE is used) how many records were deleted due to duplication of unique key values. See Section 18.2.3.33, “mysql_info()”.

If neither NULL nor NOT NULL is specified, the column is treated as though NULL had been specified.

An integer column can have the additional attribute AUTO_INCREMENT. When you insert a value of NULL (recommended) or 0 into an indexed AUTO_INCREMENT column, the column is set to the next sequence value. Typically this is value+1, where value is the largest value for the column currently in the table. AUTO_INCREMENT sequences begin with 1. Such a column must be defined as one of the integer types as described in Section 11.1.1, “Overview of Numeric Types”. (The value 1.0 is not an integer.) See Section 18.2.3.35, “mysql_insert_id()”.

As of MySQL 4.1.1, specifying the NO_AUTO_VALUE_ON_ZERO flag for the --sql-mode server option or the sql_mode system variable allows you to store 0 in AUTO_INCREMENT columns as 0 without generating a new sequence value. See Section 5.2.1, “mysqld Command-Line Options”.

Note: There can be only one AUTO_INCREMENT column per table, it must be indexed, and it cannot have a DEFAULT value. As of MySQL 3.23, an AUTO_INCREMENT column works properly only if it contains only positive values. Inserting a negative number is regarded as inserting a very large positive number. This is done to avoid precision problems when numbers “wrap” over from positive to negative and also to ensure that you do not accidentally get an AUTO_INCREMENT column that contains 0.

For MyISAM and BDB tables, you can specify an AUTO_INCREMENT secondary column in a multiple-column key. See Section 3.6.9, “Using AUTO_INCREMENT”.

To make MySQL compatible with some ODBC applications, you can find the AUTO_INCREMENT value for the last inserted row with the following query:

SELECT * FROM tbl_name WHERE auto_col IS NULL

As of MySQL 4.1, character column definitions can include a CHARACTER SET attribute to specify the character set and, optionally, a collation for the column. For details, see Chapter 10, Character Set Support. CHARSET is a synonym for CHARACTER SET.

CREATE TABLE t (c CHAR(20) CHARACTER SET utf8 COLLATE utf8_bin);

Also as of 4.1, MySQL interprets length specifications in character column definitions in characters. (Earlier versions interpret them in bytes.)

NULL values are handled differently for TIMESTAMP columns than for other column types. Before MySQL 4.1.6, you cannot store a literal NULL in a TIMESTAMP column; setting the column to NULL sets it to the current date and time. Because TIMESTAMP columns behave this way, the NULL and NOT NULL attributes do not apply in the normal way and are ignored if you specify them. On the other hand, to make it easier for MySQL clients to use TIMESTAMP columns, the server reports that such columns can be assigned NULL values (which is true), even though TIMESTAMP never actually contains a NULL value. You can see this when you use DESCRIBE tbl_name to get a description of your table.

Note that setting a TIMESTAMP column to 0 is not the same as setting it to NULL, because 0 is a valid TIMESTAMP value.

The DEFAULT clause specifies a default value for a column. With one exception, the default value must be a constant; it cannot be a function or an expression. This means, for example, that you cannot set the default for a date column to be the value of a function such as NOW() or CURRENT_DATE. The exception is that you can specify CURRENT_TIMESTAMP as the default for a TIMESTAMP column as of MySQL 4.1.2. See Section 11.3.1.2, “TIMESTAMP Properties as of MySQL 4.1”.

If a column definition includes no explicit DEFAULT value, MySQL determines the default value as follows:

If the column can take NULL as a value, the column is defined with an explicit DEFAULT NULL clause.

If the column cannot take NULL as the value, MySQL defines the column with an explicit DEFAULT clause, using the implicit default value for the column data type. Implicit defaults are defined as follows:

BLOB and TEXT columns cannot be assigned a default value.

For a given table, you can use the SHOW CREATE TABLE statement to see which columns have an explicit DEFAULT clause.

A comment for a column can be specified with the COMMENT option. The comment is displayed by the SHOW CREATE TABLE and SHOW FULL COLUMNS statements. This option is operational as of MySQL 4.1. (It is allowed but ignored in earlier versions.)

From MySQL 4.1.0 on, the attribute SERIAL can be used as an alias for BIGINT UNSIGNED NOT NULL AUTO_INCREMENT UNIQUE.

KEY is normally a synonym for INDEX. From MySQL 4.1, the key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.

In MySQL, a UNIQUE index is one in which all values in the index must be distinct. An error occurs if you try to add a new row with a key that matches an existing row. The exception to this is that if a column in the index is allowed to contain NULL values, it can contain multiple NULL values. This exception does not apply to BDB tables, for which an indexed column allows only a single NULL.

A PRIMARY KEY is a unique KEY where all key columns must be defined as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently). A table can have only one PRIMARY KEY. If you do not have a PRIMARY KEY and an application asks for the PRIMARY KEY in your tables, MySQL returns the first UNIQUE index that has no NULL columns as the PRIMARY KEY.

In the created table, a PRIMARY KEY is placed first, followed by all UNIQUE indexes, and then the non-unique indexes. This helps the MySQL optimizer to prioritize which index to use and also more quickly to detect duplicated UNIQUE keys.

A PRIMARY KEY can be a multiple-column index. However, you cannot create a multiple-column index using the PRIMARY KEY key attribute in a column specification. Doing so only marks that single column as primary. You must use a separate PRIMARY KEY(index_col_name, ...) clause.

If a PRIMARY KEY or UNIQUE index consists of only one column that has an integer type, you can also refer to the column as _rowid in SELECT statements (new in MySQL 3.23.11).

In MySQL, the name of a PRIMARY KEY is PRIMARY. For other indexes, if you do not assign a name, the index is assigned the same name as the first indexed column, with an optional suffix (_2, _3, ...) to make it unique. You can see index names for a table using SHOW INDEX FROM tbl_name. See Section 13.5.4.11, “SHOW INDEX Syntax”.

From MySQL 4.1.0 on, some storage engines allow you to specify an index type when creating an index. The syntax for the index_type specifier is USING type_name.

Example:

CREATE TABLE lookup
  (id INT, INDEX USING BTREE (id))
  ENGINE = MEMORY;

For details about USING, see Section 13.1.4, “CREATE INDEX Syntax”.

For more information about how MySQL uses indexes, see Section 7.4.5, “How MySQL Uses Indexes”.

Only the MyISAM, InnoDB, BDB, and (as of MySQL 4.0.2) MEMORY storage engines support indexes on columns that can have NULL values. In other cases, you must declare indexed columns as NOT NULL or an error results.

With col_name(length) syntax in an index specification, you can create an index that uses only the first length characters of a CHAR or VARCHAR column. Indexing only a prefix of column values like this can make the index file much smaller. See Section 7.4.3, “Column Indexes”.

The MyISAM and (as of MySQL 4.0.14) InnoDB storage engines also support indexing on BLOB and TEXT columns. When indexing a BLOB or TEXT column, you must specify a prefix length for the index. For example:

CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10)));

Prefixes can be up to 1000 bytes long (767 bytes for InnoDB tables). (Before MySQL 4.1.2, the limit is 255 bytes for all tables.) Note that prefix limits are measured in bytes, whereas the prefix length in CREATE TABLE statements is interpreted as number of characters. Be sure to take this into account when specifying a prefix length for a column that uses a multi-byte character set.

An index_col_name specification can end with ASC or DESC. These keywords are allowed for future extensions for specifying ascending or descending index value storage. Currently they are parsed but ignored; index values are always stored in ascending order.

When you use ORDER BY or GROUP BY on a TEXT or BLOB column in a SELECT, the server sorts values using only the initial number of bytes indicated by the max_sort_length system variable. See Section 11.4.3, “The BLOB and TEXT Types”.

In MySQL 3.23.23 or later, you can create special FULLTEXT indexes, which are used for full-text searches. Only the MyISAM table type supports FULLTEXT indexes. They can be created only from CHAR, VARCHAR, and TEXT columns. Indexing always happens over the entire column; partial indexing is not supported and any prefix length is ignored if specified. See Section 12.7, “Full-Text Search Functions” for details of operation.

In MySQL 4.1 or later, you can create SPATIAL indexes on spatial column types. Spatial types are supported only for MyISAM tables and indexed columns must be declared as NOT NULL. See Chapter 17, Spatial Extensions in MySQL.

In MySQL 3.23.44 or later, InnoDB tables support checking of foreign key constraints. See Chapter 15, The InnoDB Storage Engine. Note that the FOREIGN KEY syntax in InnoDB is more restrictive than the syntax presented for the CREATE TABLE statement at the beginning of this section: The columns of the referenced table must always be explicitly named. InnoDB supports both ON DELETE and ON UPDATE actions on foreign keys as of MySQL 3.23.50 and 4.0.8, respectively. For the precise syntax, see Section 15.7.4, “FOREIGN KEY Constraints”.

For other storage engines, MySQL Server parses the FOREIGN KEY and REFERENCES syntax in CREATE TABLE statements, but without further action being taken. The CHECK clause is parsed but ignored by all storage engines. See Section 1.8.5.5, “Foreign Keys”.

For MyISAM and ISAM tables, each NULL column takes one bit extra, rounded up to the nearest byte. The maximum record length in bytes can be calculated as follows:

row length = 1
             + (sum of column lengths)
             + (number of NULL columns + delete_flag + 7)/8
             + (number of variable-length columns)

delete_flag is 1 for tables with static record format. Static tables use a bit in the row record for a flag that indicates whether the row has been deleted. delete_flag is 0 for dynamic tables because the flag is stored in the dynamic row header.

These calculations do not apply for InnoDB tables, for which storage size is no different for NULL columns than for NOT NULL columns.

AUTO_INCREMENT

The initial AUTO_INCREMENT value for the table. This works for MyISAM only, for MEMORY as of MySQL 4.1.0, and for InnoDB as of MySQL 4.1.2. To set the first auto-increment value for engines that do not support the AUTO_INCREMENT table option, insert a “dummy” row with a value one less than the desired value after creating the table, and then delete the dummy row.

For engines that support the AUTO_INCREMENT table option in CREATE TABLE statements, you can also use ALTER TABLE tbl_name AUTO_INCREMENT = n to reset the AUTO_INCREMENT value.

AVG_ROW_LENGTH

An approximation of the average row length for your table. You need to set this only for large tables with variable-size records.

When you create a MyISAM table, MySQL uses the product of the MAX_ROWS and AVG_ROW_LENGTH options to decide how big the resulting table is. If you do not specify either option, the maximum size for a table is 4GB. (If your operating system does not support files that large, table sizes are constrained by the operating system limit.) If you want to keep down the pointer sizes to make the index smaller and faster and you do not really need big files, you can decrease the default pointer size by setting the myisam_data_pointer_size system variable, which was added in MySQL 4.1.2. (See Section 5.2.3, “Server System Variables”.) If you want all your tables to be able to grow above the default limit and are willing to have your tables slightly slower and larger than necessary, you may increase the default pointer size by setting this variable.

[DEFAULT] CHARACTER SET

Specify a default character set for the table. CHARSET is a synonym for this.

for CHARACTER SET.

COLLATE

Specify a default collation for the table.

CHECKSUM

Set this to 1 if you want MySQL to maintain a live checksum for all rows (that is, a checksum that MySQL updates automatically as the table changes). This makes the table a little slower to update, but also makes it easier to find corrupted tables. The CHECKSUM TABLE statement reports the checksum (MyISAM only).

COMMENT

A comment for the table, up to 60 characters long.

MAX_ROWS

The maximum number of rows you plan to store in the table. This is not a hard limit, but rather an indicator that the table must be able to store at least this many rows.

MIN_ROWS

The minimum number of rows you plan to store in the table.

PACK_KEYS

Set this option to 1 if you want to have smaller indexes. This usually makes updates slower and reads faster. Setting the option to 0 disables all packing of keys. Setting it to DEFAULT (MySQL 4.0) tells the storage engine to pack only long CHAR or VARCHAR columns. (MyISAM and ISAM only.)

If you do not use PACK_KEYS, the default is to pack only strings, but not numbers. If you use PACK_KEYS=1, numbers are packed as well.

When packing binary number keys, MySQL uses prefix compression:

This means that if you have many equal keys on two consecutive rows, all following “same” keys usually only take two bytes (including the pointer to the row). Compare this to the ordinary case where the following keys takes storage_size_for_key + pointer_size (where the pointer size is usually 4). Conversely, you get a big benefit from prefix compression only if you have many numbers that are the same. If all keys are totally different, you use one byte more per key, if the key is not a key that can have NULL values. (In this case, the packed key length is stored in the same byte that is used to mark if a key is NULL.)

PASSWORD

Encrypt the .frm file with a password. This option does not do anything in the standard MySQL version.

DELAY_KEY_WRITE

Set this to 1 if you want to delay key updates for the table until the table is closed (MyISAM only).

ROW_FORMAT

Defines how the rows should be stored. Currently this option works only with MyISAM tables. The option value can FIXED or DYNAMIC for static or variable-length row format. myisampack sets the type to COMPRESSED. See Section 14.1.3, “MyISAM Table Storage Formats”.

RAID_TYPE

The RAID_TYPE option can help you to exceed the 2GB/4GB limit for the MyISAM data file (not the index file) on operating systems that do not support big files. This option is unnecessary and not recommended for filesystems that support big files.

You can get more speed from the I/O bottleneck by putting RAID directories on different physical disks. The only allowed RAID_TYPE is STRIPED. 1 and RAID0 are aliases for STRIPED.

If you specify the RAID_TYPE option for a MyISAM table, specify the RAID_CHUNKS and RAID_CHUNKSIZE options as well. The maximum RAID_CHUNKS value is 255. MyISAM creates RAID_CHUNKS subdirectories named 00, 01, 02, ... 09, 0a, 0b, ... in the database directory. In each of these directories, MyISAM creates a file tbl_name.MYD. When writing data to the data file, the RAID handler maps the first RAID_CHUNKSIZE*1024 bytes to the first file, the next RAID_CHUNKSIZE*1024 bytes to the next file, and so on.

RAID_TYPE works on any operating system, as long as you have built MySQL with the --with-raid option to configure. To determine whether a server supports RAID tables, use SHOW VARIABLES LIKE 'have_raid' to see whether the variable value is YES.

UNION

UNION is used when you want to use a collection of identical tables as one. This works only with MERGE tables. See Section 14.2, “The MERGE Storage Engine”.

In MySQL 4.1, you must have SELECT, UPDATE, and DELETE privileges for the tables you map to a MERGE table. (Note: Originally, all tables used had to be in the same database as the MERGE table itself. This restriction has been lifted as of MySQL 4.1.1.

INSERT_METHOD

If you want to insert data into a MERGE table, you must specify with INSERT_METHOD the table into which the row should be inserted. INSERT_METHOD is an option useful for MERGE tables only. Use a value of FIRST or LAST to have inserts go to the first or last table, or a value of NO to prevent inserts. This option was introduced in MySQL 4.0.0. See Section 14.2, “The MERGE Storage Engine”.

DATA DIRECTORY, INDEX DIRECTORY

By using DATA DIRECTORY='directory' or INDEX DIRECTORY='directory' you can specify where the MyISAM storage engine should put a table's data file and index file. Note that the directory should be a full path to the directory (not a relative path).

These options work only for MyISAM tables from MySQL 4.0 on, when you are not using the --skip-symbolic-links option. Your operating system must also have a working, thread-safe realpath() call. See Section 7.6.1.2, “Using Symbolic Links for Tables on Unix” for more complete information.

All files with these extensions:

All subdirectories with names that consist of two hex digits 00-ff. These are subdirectories used for RAID tables. (These directories are not removed in versions of MySQL after 4.1, where support for RAID tables is removed. You should convert any existing RAID tables and remove these directories manually before upgrading to later MySQL versions.)

The db.opt file, if it exists.

The statement drops only TEMPORARY tables.

The statement does not end an ongoing transaction.

No access rights are checked. (A TEMPORARY table is visible only to the clien