7.8 `ALTER TABLE` syntax

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

alter_specification:
        ADD [COLUMN] create_definition [FIRST | AFTER column_name ]
  or    ADD [COLUMN] (create_definition, create_definition,...)
  or    ADD INDEX [index_name] (index_col_name,...)
  or    ADD PRIMARY KEY (index_col_name,...)
  or    ADD UNIQUE [index_name] (index_col_name,...)
  or    ALTER [COLUMN] col_name {SET DEFAULT literal | DROP DEFAULT}
  or    CHANGE [COLUMN] old_col_name create_definition
  or    MODIFY [COLUMN] create_definition
  or    DROP [COLUMN] col_name
  or    DROP PRIMARY KEY
  or    DROP INDEX index_name
  or    RENAME [AS] new_tbl_name
  or    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 7.7 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 7.7.1 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.

To use ALTER TABLE, you need select, insert, delete, update, create and drop privileges on the table.
IGNORE is a MySQL extension to ANSI SQL92. 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 ANSI SQL92, which allows only one of each clause per ALTER TABLE statement.
CHANGE col_name, DROP col_name and DROP INDEX are MySQL extensions to ANSI SQL92.
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 AS 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.
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 7.7 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 two column names 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 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 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.
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. Default value assignment is described in section 7.7 CREATE TABLE syntax.
DROP INDEX removes an index. This is a MySQL extension to ANSI SQL92.
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.
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.)
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. The syntax for them 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 5.4 Functionality missing from MySQL.

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

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.

See also See section 20.20 Problems with ALTER TABLE..

7.8 ALTER TABLE syntax

7.8 `ALTER TABLE` syntax