连接查询 select *from info,nation 形成笛卡尔积 select info.code,info.name,info.Sex,nation.name,info.birthday from info,nation where info.nation=nation.code select *from info join nation on info.nation=nation.code 关键字join 加入 联合查询 关键字：union select code,name from info union select code,name from nation 子查询 无关子查询 select *from info where nation=(select code from nation where name='汉族') 相关子查询 select * from car a where oil<(select avg(oil) from car b where b.brand=a.brand)