How to write foreign keys in mysql

How to write foreign keys in mysql?

mysql Add foreign key:

Add foreign key to the added data table:

Syntax:

alter table 表名 add constraint FK_ID foreign key(你的外键字段名) REFERENCES 外表表名(对应的表的主键字段名)；

Example :

alter table tb_active add constraint FK_ID foreign key(user_id) REFERENCES tb_user(id)
//FK_ID是外键的名称
/*
CREATE TABLE `tb_active` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`title` varchar(100) CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`content` text CHARACTER SET utf8 COLLATE utf8_unicode_ci NOT NULL,
`user_id` int(11) NOT NULL,
PRIMARY KEY (`id`),
KEY `user_id` (`user_id`),
KEY `user_id_2` (`user_id`),
CONSTRAINT `FK_ID` FOREIGN KEY (`user_id`) REFERENCES `tb_user` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1
*/

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