mysql在表中添加多个外键/增加外键/级联约束

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建表时创建外键：

CREATE TABLE`xh` ( `id` int(100) unsigned NOT NULL AUTO_INCREMENT COMMENT , `cl_id` smallint(3) unsigned NOT NULL COMMENT, `title` varchar(100) COLLATE utf8_unicode_ci NOT NULL COMMENT , `details` text COLLATE utf8_unicode_ci NOT NULL COMMENT , `date` datetime NOT NULL COMMENT , `au_id` smallint(6) unsigned NOT NULL COMMENT , `click` int(100) unsigned NOT NULL DEFAULT '0' COMMENT , `reco` int(100) unsigned NOT NULL DEFAULT '0' COMMENT, PRIMARY KEY (`id`), KEY `fk_class` (`cl_id`), CONSTRAINT `fk_class`FOREIGN KEY  (`cl_id`)  REFERENCES  `fl` (`id`), KEY `fk_author` (`au_id`), CONSTRAINT `fk_author` FOREIGN KEY (`au_id`) REFERENCES `author` (`id`)) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci

建表后创建外键：

Alter Table `ym` Add Constraint `fk_author` Foreign Key (`au_id`) References `author` (`id`);

查看表结构：

Show Create Table `ym`;

4.级联：在末尾可加上（可单独添加，也可全部添加）：

ON UPDATE CASCADE（级联更新）ON DELETE CASCADE（级联删除）

比如：

ALTER TABLE `ym`  ADD  CONSTRAINT  `fk_author`  FOREIGN  KEY  (`au_id`)

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