6.5.4 ALTER TABLE Syntax

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

alter_specification:
    ADD [COLUMN] create_definition [FIRST | AFTER column_name ]
  | ADD [COLUMN] (create_definition, create_definition,...)
  | ADD INDEX [index_name] (index_col_name,...)
  | ADD PRIMARY KEY (index_col_name,...)
  | ADD UNIQUE [index_name] (index_col_name,...)
  | ADD FULLTEXT [index_name] (index_col_name,...)
  | 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 create_definition
           [FIRST | AFTER column_name]
  | MODIFY [COLUMN] create_definition [FIRST | AFTER column_name]
  | DROP [COLUMN] col_name
  | DROP PRIMARY KEY
  | DROP INDEX index_name
  | DISABLE KEYS
  | ENABLE KEYS
  | RENAME [TO] new_tbl_name
  | ORDER BY col
  | table_options

ALTER TABLE allows 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. See section 6.5.3 CREATE TABLE 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 6.5.3.1 Silent Column Specification Changes. For example, if you try to change a VARCHAR column to CHAR, MySQL will still use VARCHAR if the table contains other variable-length columns.

ALTER TABLE works by making a temporary copy of the original table. The alteration is performed on the copy, then the original table is deleted and the new one is renamed. This is done in such a way that all updates are automatically redirected to the new table without any failed updates. 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.

Note that if you use any other option to ALTER TABLE than RENAME, MySQL will always create a temporary table, even if the data wouldn't strictly need to be copied (like when you change the name of a column). We plan to fix this in the future, but as one doesn't normally do ALTER TABLE that often this isn't that high on our TODO. For MyISAM tables, you can speed up the index recreation part (which is the slowest part of the recreation process) by setting the myisam_sort_buffer_size variable to a high value.

• To use ALTER TABLE, you need ALTER, INSERT, and CREATE privileges on the table.
• IGNORE is a MySQL extension to SQL-92. It controls how ALTER TABLE works if there are duplicates on unique keys in the new table. If IGNORE isn't specified, the copy is aborted and rolled back. If IGNORE is specified, then for rows with duplicates on a unique key, only the first row is used; the others are deleted.
• You can issue multiple ADD, ALTER, DROP, and CHANGE clauses in a single ALTER TABLE statement. This is a MySQL extension to SQL-92, which allows only one of each clause per ALTER TABLE statement.
• CHANGE col_name, DROP col_name, and DROP INDEX are MySQL extensions to SQL-92.
• MODIFY is an Oracle extension to ALTER TABLE.
• The optional word COLUMN is a pure noise word and can be omitted.
• If you use ALTER TABLE tbl_name RENAME TO new_name without any other options, MySQL simply renames the files that correspond to the table tbl_name. There is no need to create the temporary table. See section 6.5.5 RENAME TABLE Syntax.
• create_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 6.5.3 CREATE TABLE Syntax.
• You can rename a column using a CHANGE old_col_name create_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 Version 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 part of the column (for instance, if you have an index on the first 10 characters of a VARCHAR column), you cannot make the column shorter than the number of characters that are indexed.
• When you change a column type using CHANGE or MODIFY, MySQL tries to convert data to the new type as well as possible.
• In MySQL Version 3.22 or later, you can use FIRST or ADD ... 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 Version 4.0.1, you can also use the FIRST and AFTER keywords in CHANGE or MODIFY.
• 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 6.5.3 CREATE TABLE Syntax.
• DROP INDEX removes an index. This is a MySQL extension to SQL-92. See section 6.5.8 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. If no such index exists, it 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, this is stored before any not 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 will not remain in this order after inserts and deletes. In some cases, it may make sorting easier for MySQL if the table is in order by the column that you wish to order it by later. 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 may be able to get higher performance.
• If you use ALTER TABLE on a MyISAM table, all non-unique indexes are created in a separate batch (like in REPAIR). This should make ALTER TABLE much faster when you have many indexes.
• Since MySQL 4.0 the above feature can be activated explicitly. ALTER TABLE ... DISABLE KEYS makes MySQL to stop updating non-unique indexes for MyISAM table. ALTER TABLE ... ENABLE KEYS then should be used to recreate missing indexes. As MySQL does it with a special algorithm which is much faster then inserting keys one by one, disabling keys could give a considerable speedup on bulk inserts.
• With the C API function mysql_info(), 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.
• The FOREIGN KEY, CHECK, and REFERENCES clauses don't actually do anything, except for InnoDB type tables which support ADD CONSTRAINT FOREIGN KEY (...) REFERENCES ... (...). See section 7.5 InnoDB Tables. The syntax for other table types is provided only for compatibility, to make it easier to port code from other SQL servers and to run applications that create tables with references. See section 1.8.4 MySQL Differences Compared To SQL-92.

Here is an example that shows some of the uses of ALTER TABLE. We begin with a table t1 that is created as shown here:

mysql> CREATE TABLE t1 (a INTEGER,b CHAR(10));

To rename the table from t1 to t2:

mysql> 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:

mysql> ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);

To add a new TIMESTAMP column named d:

mysql> ALTER TABLE t2 ADD d TIMESTAMP;

To add an index on column d, and make column a the primary key:

mysql> ALTER TABLE t2 ADD INDEX (d), ADD PRIMARY KEY (a);

To remove column c:

mysql> ALTER TABLE t2 DROP COLUMN c;

To add a new AUTO_INCREMENT integer column named c:

mysql> ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT,
           ADD INDEX (c);

Note that we indexed c, because AUTO_INCREMENT columns must be indexed, and also that we declare c as NOT NULL, because indexed columns cannot be NULL.

When you add an AUTO_INCREMENT column, column values are filled in with sequence numbers for you automatically. You can set the first sequence number by executing SET INSERT_ID=# before ALTER TABLE or using the AUTO_INCREMENT = # table option. See section 5.5.6 SET Syntax.

With MyISAM tables, if you don't change the AUTO_INCREMENT column, the sequence number will not be affected. If you drop an AUTO_INCREMENT column and then add another AUTO_INCREMENT column, the numbers will start from 1 again.

See section A.7.1 Problems with ALTER TABLE..

User Comments

Suggestion: I've found that you need to refresh
(meaning quit and restart) the MySQLManager
v1.0.2 in order to see ALTER changes.

I've found that on Windows somtimes Alter doesn't
work like it should... it gives errors...

I made it work, restarting the MySQL.
(Everything works fine with selects, uptes and
inserts... just this alter gives errors sometimes)

I made an ALTER TABLE __ MODIFY a column that
was part
of an INDEX and after the command was completed
my server daemon was stuck on 99% CPU used

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;

The DISABLE KEYS and ENABLE KEYS statements
require the user to have the INDEX permission in
addition to the stated ALTER, INSERT, and CREATE.

Oh, I've forgotten: Be Aware at the foreign Keys! The foreign keys can not be deleted by now. I hope MySQL developers will continue in development of that to. You can only completely drop the table where your foreign key is set, afterwards create the table again (with or without the foreign keys) and then (if not in CREATE TABLE statement) recreate your new foreign keys with the ALTER TABLE statement.

ALTER TABLE on Windows Version of MySQL many times returns problem while the table might be open with some of client connections. But same on Linux Version runs properly. On Windows after receiving error like unable to rename or open TABLE.* file, MySQL has to be restarted, after restarting MySQL everything works well.

I received such problems with 3.23 final version frequently but on Linux it never gave such problem, I mostly used phpMyAdmin.

Hope the future versions will improve. Rest mySQL is great! pretty fast and perfect, after having mysql control center, its easy to work. I used to avoid mysql due to no good GUI for it !!

- Akash Kava
NeuroSpeech

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