10.17 (下午)开课一个月零十三天 (高级查询)
1.连接查询
select * from 表1,表2
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'要修改的列名',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)