1、Case 子查询连接查询

复制代码 代码如下:

select * from score

create database demo

use demo

create 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 MySchool

select * 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 <90 then ' 良 '

when english >=70 and english < 80 then ' 中 '

when english >= 60 and english < 70 then ' 可 '

else ' 差 '

end as '成绩 '

from score

order 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

end

from 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 < 0 then abs ( amount)

else 0

end as '支出 '

from test

--结果如下

复制代码 代码如下:

-- 有一张表student0 ，记录学生成绩

use demo

CREATE 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 student0

select [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 student0

group by [name]

复制代码 代码如下:

-- 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