12.自关联

-- 查询山东省有哪些城市

--select * from areas as provinve innser join areas as city on  city.pid = province.aid having province.atitle = "山东省"

--select provinve.atitle,city.atitle from areas as provinve innser join areas as city on  city.pid = province.aid having province.atitle = "山东省"

--子查询

--

--取消外键

show create table goods; // 可以看到外键名称

alter table goods drop foreign 外键名称