case 嵌套查询与连接查询你需要懂得

Case 子查询连接查询在sql开发中很常用的一个查询功能,感兴趣的你可不要错过本文，接下来为你详细介绍下

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 when english >=70 and english when english >= 60 and english 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 else 0
end as ‘支出 ‘
from test
–结果如下

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]

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