Mysql外键
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mysql在表中添加多个外键/增加外键/级联约束
1. 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 2. 建表后创建外键： Alter Table `ym` Add Constraint `fk_author` Foreign Key (`au_id`) References `author` (`id`); 3. 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`) REFERENCES `author` (`id`) ON UPDATE CASCADE
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