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  • 21.增删改表的字段

    21.1 命令语法及默认添加演示

    1、命令语法:alter table <表名> add <字段> <类型> 其他
    2、测试表数据:
    mysql> use oldboy;
    Database changed
    mysql> show create table testG;
    *************************** 1. row ***************************
    Table: test
    Create Table: CREATE TABLE `test` (
    `id` int(4) NOT NULL DEFAULT '0',
    `age` tinyint(2) DEFAULT NULL,
    `name` varchar(16) DEFAULT NULL,
    `shouji` char(11) DEFAULT NULL,
    PRIMARY KEY (`id`),
    KEY `index_name` (`name`)
    ) ENGINE=MyISAM DEFAULT CHARSET=utf8
    1 row in set (0.09 sec)
    mysql> desc test;
    +--------+-------------+------+-----+---------+-------+
    | Field | Type | Null | Key | Default | Extra |
    +--------+-------------+------+-----+---------+-------+
    | id | int(4) | NO | PRI | 0 | |
    | age | tinyint(2) | YES | | NULL | |
    | name | varchar(16) | YES | MUL | NULL | |
    | shouji | char(11) | YES | | NULL | |
    +--------+-------------+------+-----+---------+-------+
    4 rows in set (0.13 sec)
    3、实践案例
    例如:在表 test 中添加字段 sex
    4、执行的命令演示
    添加性别列,默认语句
    mysql> desc test;
    +--------+-------------+------+-----+---------+-------+
    | Field | Type | Null | Key | Default | Extra |
    +--------+-------------+------+-----+---------+-------+
    | id | int(4) | NO | PRI | 0 | |
    | age | int(4) | YES | | NULL | |
    | shouji | char(11) | YES | | NULL | |
    | sex | char(4) | YES | | NULL | |
    +--------+-------------+------+-----+---------+-------+
    5 rows in set (0.00 sec)
    5、 改变字段
    语法:
    alter table <表名> CHANGE [COLUMN] old_col_name new_col_name column_definition
    6、 修改字段类型
    mysql> desc test;
    +--------+-------------+------+-----+---------+-------+
    | Field | Type | Null | Key | Default | Extra |
    +--------+-------------+------+-----+---------+-------+
    | id | int(4) | NO | PRI | 0 | |
    | name | varchar(16) | YES | MUL | NULL | |
    | age | int(4) | YES | | NULL | | 修改前
    | shouji | char(11) | YES | | NULL | |
    | sex | char(4) | YES | | NULL | |
    +--------+-------------+------+-----+---------+-------+
    5 rows in set (0.00 sec)
    mysql> alter table test modify age char(4);
    Query OK, 2 rows affected (0.06 sec)
    Records: 2 Duplicates: 0 Warnings: 0
    mysql> desc test;
    +--------+-------------+------+-----+---------+-------+
    | Field | Type | Null | Key | Default | Extra |
    +--------+-------------+------+-----+---------+-------+
    | id | int(4) | NO | PRI | 0 | |
    | name | varchar(16) | YES | MUL | NULL | |
    | age | char(4) | YES | | NULL | | 修改后
    | shouji | char(11) | YES | | NULL | |
    | sex | char(4) | YES | | NULL | |
    +--------+-------------+------+-----+---------+-------+
    5 rows in set (0.00 sec)​
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  • 原文地址:https://www.cnblogs.com/hackerlin/p/12539980.html
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