case 嵌套查询与连接查询你需要懂得

-- 子查询将一个查询语句做为一个结果集供其他 SQL 语句使用，就像使用普通的表一样，
-- 被当作结果集的查询语句被称为子查询。所有可以使用表的地方几乎都可以使用子查询来代替。
use myschool
select sName from ( select * from student ) as t
select 1,( select sum ( english) from score ) as ' 和 ',( select avg ( sAge) from student ) as ' 平均年龄 '
-- 查询高一一班所有的学生
select * from student where sClassId =
( select cId from class where cName = '高一一班 ' )
-- 查询高一一班 高二一班所有的学生
-- 子查询返回的值不止一个。当子查询跟随在 = 、!= 、 <、 <= 、> 、 >= 之后
-- 子查询跟在比较运算符之后，要求子查询只返回一个值
-- 如果子查询是多行单列的子查询，这样的子查询的结果集其实是一个集合。可以使用 in 关键字代替 =号
select * from student where sClassId =
( select cId from class where cName in ( '高一一班 ' ,' 高二一班 '))
select * from student where sClassId in
( select cId from class where cName in ( '高一一班 ' ,' 高二一班 '))
-- 查询刘关张的成绩
select * from score where studentId in
( select sId from student where sName in ( '刘备 ' ,' 关羽 ', ' 张飞' ))
-- 删除刘关张
delete from score where studentId in
( select sId from student where sName in ( '刘备 ' ,' 关羽 ', ' 张飞' ))

-- 实现分页
-- 最近入学的个学生
select top 3 * from student
order by sId desc
-- 查询第到个学生
select top 3 * from student
where sId not in ( select top 3 sId from student order by sId desc)
order by sId desc
-- 查询到的学生
select top 3 * from student
where sId not in ( select top 6 sId from student order by sId desc)
order by sId desc
-- 上面是sql 2000 以前的实现方式。 SQLServer2005 后增加了Row_Number 函数简化实现。
--sql 2005 中的分页
select * from
( select row_number () over (order by sId desc ) as num,* from student ) as t
where num between 1 and 3
select * from
( select row_number () over (order by sId desc ) as num,* from student ) as t
where num between 4 and 6
select * from
( select row_number () over (order by sId desc ) as num,* from student ) as t
where num between 7 and 9
select * from
( select row_number () over (order by sId desc ) as num,* from student ) as t
where num between 3 *( 3- 1 ) + 1 and 3 *3
-- 表连接
-- 交叉连接cross join
select * from student
cross join class
-- 内连接inner join...on...
select * from student
inner join class on sClassId = cId
select * from class
-- 查询所有学生的姓名、年龄及所在班级
select sName , sAge, cName ,sSex from student
inner join class on sClassId = cId
where sSex =' 女 '
-- 查询年龄超过岁的学生的姓名、年龄及所在班级
select sName , sAge, cName from class
inner join student on sClassId = cId
where sAge > 20
-- 外连接
--left join...on...
select sName , sAge, cName from class