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高级查询
1.连接查询(对列的扩展)
第一种形式
select * from Info,Nation #会形成笛卡尔积
select * from Info,Nation where Info.Nation = Nation.Code #加入筛选条件
select Info.Code,Info.Name,Sex,Nation.Name from Info,Nation where Info.Nation = Nation.Code
#查询指定列
select Info.Code as ‘代号‘,Info.Name as ‘姓名‘,Sex as ‘性别‘,Nation.Name as ‘民族‘,Birthday as ‘生日‘ from Info,Nation where Info.Nation = Nation.Code
#换表头
第二种形式:
select * from Info join Nation #join连接,形成笛卡尔积 查询很慢
select * from Info join Nation on Info.Nation = Nation.Code #join关键字
2.联合查询(对行的扩展)
select * from Info where Nation = ‘n002‘
union #关键字
select * from Info where Code = ‘p002‘
3.子查询(无关子查询) 如果子查询语句可以单独拿出来执行,就是无关查询
在一个sql语句中有两个子查询,其中一个a查询的结果作为另一个b的
select查询条件,a成为里层查询,b为外层查询或父查询
查询民族为‘汉族‘的人员信息:
select * from Info where Nation = ( select Code from Nation where Name = ‘汉族‘)
查询民族为 ‘汉族‘ 或者 ‘回族‘ 的人员信息
select * from Info where Nation in (select Code from Nation where Name = ‘汉族‘ or Name = ‘回族‘)
select * from Info where Nation not in (select Code from Nation where Name = ‘汉族‘ or Name = ‘回族‘)
#in 关键字 not in不在表里
4.子查询(相关子查询)
查询同一系列的 油耗 要比平均油耗低的汽车信息
子查询
select avg(oil) from Car where Brand = ‘‘
父查询
select * from Car where oil<平均油耗
select * from Car a where a.oil < (select avg(b.oil) from Car b where b.Brand = a.Brand)
#b.Brand 是 查询条件,a.Brand 是 逐条的信息
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原文地址：http://www.cnblogs.com/ds-3579/p/5470952.html
