MySQL删除外键定义的方法

MySQL外键在定以后，如果我们不再需要这个外键，可以进行删除操作，下面就为您介绍MySQL删除外键定义的方法，供您参考。

不知道大家有没有发现，在定义外键的时候articles.member_id外键比articles.category_id子句多了一个CONSTRAINT fk_member ？
这个fk_member就是用来实现MySQL删除外键用的，如下所示：

mysql> ALTER TABLE articles DROP FOREIGN KEY fk_member;
Query OK, 1 row affected (0.25 sec)
Records: 1    Duplicates: 0    Warnings: 0

这样articles.member_id外键定义就被删除了，但是如果定义时没有指定CONSTRAINT fk_symbol （即外键符号）时该怎么实现MySQL删除外键呢？别急，没有指定时，MySQL会自己创建一个，可以通过以下命令查看：

mysql> SHOW CREATE TABLE articles;
+———-+————————————+
| Table      | Create Table                         |
+———-+————————————+
| articles | CREATE TABLE `articles` (
    `article_id` int(11) unsigned NOT NULL auto_increment,
    `category_id` tinyint(3) unsigned NOT NULL,
    `member_id` int(11) unsigned NOT NULL,
    `title` varchar(255) NOT NULL,
    PRIMARY KEY    (`article_id`),
    KEY `category_id` (`category_id`),
    KEY `member_id` (`member_id`),
    CONSTRAINT `articles_ibfk_1` FOREIGN KEY (`category_id`) REFERENCES `categories` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1            |
+———-+————————————+
1 row in set (0.01 sec)

可以看出articles.category_id的外键符号为articles_ibfk_1，因为就可以执行以下命令实现MySQL删除外键定义：

mysql> ALTER TABLE articles DROP FOREIGN KEY articles_ibfk_1;  注意大小写
Query OK, 1 row affected (0.66 sec)
Records: 1    Duplicates: 0    Warnings: 0
mysql> ALTER TABLEarticles DROP column member_id;
--------------------------------------------------------------------------
删除索引

show index 发现有和外键同名的索引存在 mysql> show index from CORE_FUNCTION; +---------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +---------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | CORE_FUNCTION | 0 | PRIMARY | 1 | FUNCTION_ID | A | 0 | NULL | NULL | | BTREE | | | CORE_FUNCTION | 1 | FKDB8410785D1C928B | 1 | PARENT | A | 0 | NULL | NULL | YES | BTREE | | +---------------+------------+--------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 2 rows in set (0.01 sec)

删除掉索引 mysql> alter table CORE_FUNCTION drop index FKDB8410785D1C928B; Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0

再show keys看,外键不见了 mysql> show keys from CORE_FUNCTION; +---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | +---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ | CORE_FUNCTION | 0 | PRIMARY | 1 | FUNCTION_ID | A | 0 | NULL | NULL | | BTREE | | +---------------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+ 1 row in set (0.00 sec)

mysql alter 语句用法,添加、修改、删除字段等

 

//主键549830479

   alter table tabelname add new_field_id int(5) unsigned default 0 not null auto_increment ,add primary key (new_field_id);

//增加一个新列549830479

   alter table t2 add d timestamp;
alter table infos add ex tinyint not null default '0';

//删除列549830479

   alter table t2 drop column c;

//重命名列549830479

   alter table t1 change a b integer;

//改变列的类型549830479

   alter table t1 change b b bigint not null;
alter table infos change list list tinyint not null default '0';

//重命名表549830479

   alter table t1 rename t2;

加索引549830479

   mysql> alter table tablename change depno depno int(5) not null;
mysql> alter table tablename add index 索引名 (字段名1[，字段名2 …]);
mysql> alter table tablename add index emp_name (name);

加主关键字的索引549830479

mysql> alter table tablename add primary key(id);

加唯一限制条件的索引549830479

  mysql> alter table tablename add unique emp_name2(cardnumber);

删除某个索引549830479

   mysql>alter table tablename drop index emp_name;

修改表：549830479

增加字段：549830479

   mysql> ALTER TABLE table_name ADD field_name field_type;

修改原字段名称及类型：549830479

   mysql> ALTER TABLE table_name CHANGE old_field_name new_field_name field_type;

删除字段：549830479

   mysql> ALTER TABLE table_name DROP field_name;