相关mysql触发器功能的问题

一个树目录结构的数据库 module(id,name,fatherId)
我想写一个触发器，功能是：删除一个节点，同时删除该节点下的多有子节点。
问题：我下面的句子不能执行，报错。
`CREATE

TRIGGER DeleteModuleNode AFTER DELETE ON module
    FOR EACH ROW 
BEGIN
    IF((SELECT COUNT(id) FROM OLD) > 0) THEN
        DELETE FROM module WHERE fatherId IN (SELECT id FROM OLD);
    END IF;
END;`

此行报错：DELETE FROM module WHERE fatherId IN (SELECT id FROM OLD);
错误码: 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 '' at line 6