数据库_CRUD操作之读取数据之高级读取

高级读取

1.连接查询

select * from info,nation

形成笛卡尔积

select * from info,nation where info.nation=nation.code

select Info.code,Info.name,Info.sex,Nation.name as '民族',Info.birthday from Info,Nation where Info.nation=Nation.code

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
子查询查询的结果作为父查询的条件

（1）无关子查询：子查询执行的时候和父查询没有关系
查民族为'汉族'的所有学生信息
select * from Info where nation=(select code from nation where name='汉族')

查询生产厂商为'一汽大众'的所有汽车信息
select * from car where brand=()
select brand_code from brand where prod_code=()
select prod_code from productor where prod_name='一汽大众'

select * from car where brand in(select brand_code from brand where prod_code=(select prod_code from productor where prod_name='一汽大众'))

（2）相关子查询
子查询在执行的时候需要用到父查询的内容

查询汽车表中，汽车油耗小于该系列平均油耗的所有汽车信息

select * from car where oil<(该系列平均油耗)
select avg(oil) from car where brand =(该系列)

select * from car a where oil<(select avg(oil) from car b where b.brand =a.brand)