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13.1.4 ALTER TABLE Syntax ALTER TABLE Examples
    [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_type]
        [index_type] (index_col_name,...) [index_type]
  | ADD [CONSTRAINT [symbol]]
        UNIQUE [INDEX|KEY] [index_name]
        [index_type] (index_col_name,...) [index_type]
        (index_col_name,...) [index_type]
  | 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]
  | MODIFY [COLUMN] col_name column_definition
        [FIRST | AFTER col_name]
  | DROP [COLUMN] col_name
  | DROP {INDEX|KEY} index_name
  | DROP FOREIGN KEY fk_symbol
  | RENAME [TO|AS] 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]

    col_name [(length)] [ASC | DESC]


    table_option [[,] table_option] ...  (see CREATE TABLE options)

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.

Following the table name, specify the alterations to be made. If none are given, ALTER TABLE does nothing.

The syntax for many of the permissible alterations is similar to clauses of the CREATE TABLE statement. See Section 13.1.10, “CREATE TABLE Syntax”, for more information.

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 a list of all table options and a description of each, see Section 13.1.10, “CREATE TABLE Syntax”. However, ALTER TABLE ignores the DATA DIRECTORY and INDEX DIRECTORY table options.

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, “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, “Silent Column Specification Changes”.

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, “SHOW WARNINGS Syntax”.

For information on troubleshooting ALTER TABLE, see Section B.5.6.1, “Problems with ALTER TABLE”.

Storage, Performance, and Concurrency Considerations

In most cases, ALTER TABLE makes 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. 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.

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 without making a copy. (You can also use the RENAME TABLE statement to rename tables. See Section 13.1.20, “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 index re-creation (the slowest part of the alteration process) by setting the myisam_sort_buffer_size system variable to a high value.

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