PHP 高级查询
高级查询:
1.连接查询
select * from Info,Nation #得出的结果称为笛卡尔积 select * from Info,Nation where Info.Nation = Nation.Code
join on连接
select * from Info join Nation #join连接 select * from Info join Nation on Info.Nation = Nation.Code
2.联合查询
select Code,Name from Info union select Code,Name from Nation
3.子查询
1)无关子查询
select Code from Nation where Name = '汉族' #去Nation表中查询汉族的民族代号
select * from Info where Nation = (民族代号) #在Info表中查询民族代号为上一个查询结果的所有信息
select * from Info where Nation = (select Code from Nation where Name = '汉族')
子查询查询的结果被父查询使用,子查询可以单独执行的成为无关子查询
2)相关子查询
select * from Car where Oil<(该系列的平均油耗) #查询油耗小于该系列平均油耗的
select avg(Oil) from Car where Brand = "值" #查询某系列的平均油耗
select * from Car a where Oil<(select avg(Oil) from Car b where b.Brand = a.Brand)