Mariadb 简明教程
MariaDB - Alter Command
ALTER 命令提供了一种修改现有表结构的方法，它表示移除或添加列、修改索引、更改数据类型或更改名称等修改。当元数据锁处于活动状态时，ALTER 也会等待应用更改。
The ALTER command provides a way to change an existing table’s structure, meaning modifications like removing or adding columns, modifying indices, changing data types, or changing names. ALTER also waits to apply changes when a metadata lock is active.
Using ALTER to Modify Columns
ALTER 与 DROP 结合使用，可以移除现有列。但是，如果该列是唯一剩余列，它将失败。
ALTER paired with DROP removes an existing column. However, it fails if the column is the only remaining column.
查看下面给出的示例 −
Review the example given below −
mysql> ALTER TABLE products_tbl DROP version_num;使用 ALTER…ADD 语句添加列 −
Use an ALTER…ADD statement to add columns −
mysql> ALTER TABLE products_tbl ADD discontinued CHAR(1);使用关键字 FIRST 和 AFTER 来指定列放置位置 −
Use the keywords FIRST and AFTER to specify placement of the column −
ALTER TABLE products_tbl ADD discontinued CHAR(1) FIRST;
ALTER TABLE products_tbl ADD discontinued CHAR(1) AFTER quantity;请注意，关键字 FIRST 和 AFTER 仅适用于 ALTER…ADD 语句。此外，必须删除表，然后再添加表才能重新定位。
Note the FIRST and AFTER keywords only apply to ALTER…ADD statements. Furthermore, you must drop a table and then add it in order to reposition it.
使用 ALTER 语句中的 MODIFY 或 CHANGE 子句来更改列定义或名称。这些子句具有类似效果，但使用完全不同的语法。
Change a column definition or name by using the MODIFY or CHANGE clause in an ALTER statement. The clauses have similar effects, but utilize substantially different syntax.
请查看以下给出的 CHANGE 示例 −
Review a CHANGE example given below −
mysql> ALTER TABLE products_tbl CHANGE discontinued status CHAR(4);在使用 CHANGE 的语句中，指定原始列，然后指定将替换它的新列。请查看以下 MODIFY 示例 −
In a statement using CHANGE, specify the original column and then the new column that will replace it. Review a MODIFY example below −
mysql> ALTER TABLE products_tbl MODIFY discontinued CHAR(4);ALTER 命令还允许更改默认值。请查看示例 −
The ALTER command also allows for changing default values. Review an example −
mysql> ALTER TABLE products_tbl ALTER discontinued SET DEFAULT N;还可以使用与 DROP 子句配对的方式删除默认约束 −
You can also use it to remove default constraints by pairing it with a DROP clause −
mysql> ALTER TABLE products_tbl ALTER discontinued DROP DEFAULT;