高级查询：

    一：多表连接

    1.select Info.Code,Info.Name,Nation.Name from Info,Nation where Info.Nation = Nation.Code

    select * from Info,Nation

    2.join连接

    select * from Info join Nation on Info.Nation = Nation.Code

    

    二：多表联合

    select * from Info where Code='p001'union select * from Info where Nation='n001'

    三：子查询（无关子查询）

    select * from Info where Nation = (select Code from Nation where Name='汉族')

    select * from Info where Nation in (select Code from Nation where Name='汉族' or Name='苗族')

    四：子查询（相关子查询）

    select * from Car a  where a.Oil <(select avg(Oil) from Car b where b.Brand = a.Brand)