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  

1. 建表后创建外键：

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

1. 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`)