ALTER TABLE 的使用

[谁学逆向工程]

1、图中的 restrict cascade 是什么意思，怎么用？
2、方括号中的内容是可以省略的吗？ 我按省略的做了，但是提示错误。

mysql> DESCRIBE X;
+-------+---------+------+-----+---------+-------+
| Field | Type    | Null | Key | Default | Extra |
+-------+---------+------+-----+---------+-------+
| id    | int(11) | NO   |     | NULL    |       |
+-------+---------+------+-----+---------+-------+
1 row in set (0.01 sec)

mysql> ALTER TABLE X id char(50);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that
corresponds to your MySQL server version for the right syntax to use near 'id ch
ar(50)' at line 1

[rucypli]

alter table test1 add b int;
alter table test1 drop b;

[WWWWA]

alter table b modify id char(50);

[ACMAIN_CHM]

[Quote]13.1.2. ALTER TABLE语法
ALTER [IGNORE] TABLE tbl_name
alter_specification [, alter_specification] ...

alter_specification:
ADD [COLUMN] column_definition [FIRST | AFTER col_name ]
| ADD [COLUMN] (column_definition,...)
| ADD INDEX [index_name] [index_type] (index_col_name,...)
| ADD [CONSTRAINT [symbol]]
PRIMARY KEY [index_type] (index_col_name,...)
| ADD [CONSTRAINT [symbol]]
UNIQUE [index_name] [index_type] (index_col_name,...)
| ADD [FULLTEXT|SPATIAL] [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 column_definition
[FIRST|AFTER col_name]
| MODIFY [COLUMN] column_definition [FIRST | AFTER col_name]
| DROP [COLUMN] col_name
| DROP PRIMARY KEY
| DROP INDEX index_name
| DROP FOREIGN KEY fk_symbol
| DISABLE KEYS
| ENABLE KEYS
| RENAME [TO] new_tbl_name
| ORDER BY col_name
| CONVERT TO CHARACTER SET charset_name [COLLATE collation_name]
| [DEFAULT] CHARACTER SET charset_name [COLLATE collation_name]
| DISCARD TABLESPACE
| IMPORT TABLESPACE
| table_options
| partition_options
| ADD PARTITION partition_definition
| DROP PARTITION partition_names
| COALESCE PARTITION number
| REORGANIZE PARTITION partition_names INTO (partition_definitions)
| ANALYZE PARTITION partition_names
| CHECK PARTITION partition_names
| OPTIMIZE PARTITION partition_names
| REBUILD PARTITION partition_names
| REPAIR PARTITION partition_names

[/Quote]

[氧气网航]

ALTER TABLE X id char(50) not null

[回南山种豆]

restrict cascade 是在支持外键的引擎里用的，表示当更改父表的字段时是拒绝更改，还是级联给该，中括号里面的表示可选的，有时候可能会用到！