post表和categories表我已经定义为使用InnoDB引擎；
post表中的post_category我已经建立了索引（INDEX）；

但在post表中使用sql语句'FOREIGN KEY (`post_category`) REFERENCES categories (`cat_id`) ON DELETE CASCADE' 定义外键时还是出现如下错误

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'FOREIGN KEY (`post_category`) REFERENCES categories (`cat_id`) ON DELETE CASCADE' at line 1