嵌套查询与连接查询

1、Case 子查询连接查询select * from scorecreate database demouse democreate table [user](       [uId] int identity( 1 ,1 ) primary key,       [name] varchar ( 50),       [level] int   --1骨灰大虾菜鸟)insert into [user] ( name, level ) values (' 犀利哥 ', 1 )insert into [user] ( name, level ) values (' 小月月 ', 2 )insert into [user] ( name, level ) values (' 芙蓉姐姐 ', 3 )--case end 单值判断 相当于 switch case--then 后面的返回值类型必须一致select [name] ,       case [level]             when 1 then '骨灰 '             when 2 then '大虾 '             when 3 then '菜鸟 '       end as '等级 'from [user]use MySchoolselect * from score--case end 第二种用法，相当于多重 if 语句select studentId ,       case             when english >=90 then ' 优 '             when english >=80 and english             when english >=70 and english             when english >= 60 and english             else ' 差 '       end as '成绩 'from scoreorder by english-- 表中有A B C 三列 ,用 SQL 语句实现：当 A列大于 B 列时选择A 列否则选择 B 列，当B 列大于 C列时选择 B 列否则选择 C列。select       case             when a > b then a             else b       end ,       case                when b > c then b             else c       endfrom T-- 练习create table test(       number varchar ( 10),       amount int)insert into test( number ,amount ) values ( 'RK1', 10 )insert into test( number ,amount ) values ( 'RK2', 20 )insert into test( number ,amount ) values ( 'RK3',- 30 )insert into test( number ,amount ) values ( 'RK4',- 10 )select number ,       case             when amount > 0 then amount             else 0       end as '收入 ' ,       case             when amount             else 0       end as '支出 'from test--结果如下-- 有一张表student0 ，记录学生成绩use demoCREATE TABLE student0 ( name nvarchar (10 ), subject nvarchar (10 ), result int )INSERT INTO student0 VALUES (' 张三 ', ' 语文' , 80)INSERT INTO student0 VALUES (' 张三 ', ' 数学' , 90)INSERT INTO student0 VALUES (' 张三 ', ' 物理' , 85)INSERT INTO student0 VALUES (' 李四 ', ' 语文' , 85)INSERT INTO student0 VALUES (' 李四 ', ' 数学' , 92)INSERT INTO student0 VALUES (' 李四 ', ' 物理' ,null)select * from student0select [name] ,       isnull (sum ( case subject             when ' 语文 ' then result       end ),0 ) as '语文 ' ,       isnull (sum ( case subject             when ' 数学 ' then result       end ),0 ) as '数学 ' ,       isnull (sum ( case subject             when ' 物理 ' then result       end ),0 ) as '物理 'from student0group by [name]-- 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 myschoolselect sName from ( select * from student ) as tselect 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 studentorder by sId desc-- 查询第到个学生select top 3 * from studentwhere 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 studentwhere 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 twhere num between 1 and 3select * from( select row_number () over (order by sId desc ) as num,* from student ) as twhere num between 4 and 6select * from( select row_number () over (order by sId desc ) as num,* from student ) as twhere num between 7 and 9select * from( select row_number () over (order by sId desc ) as num,* from student ) as twhere num between 3 *( 3- 1 ) + 1 and 3 *3-- 表连接-- 交叉连接cross joinselect * from studentcross join class-- 内连接inner join...on...select * from studentinner join class on sClassId = cIdselect * from class-- 查询所有学生的姓名、年龄及所在班级select sName , sAge, cName ,sSex from studentinner join class on sClassId = cIdwhere 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 classinner join student on sClassId = cIdwhere sAge > 20-- 外连接--left join...on...select sName , sAge, cName from class