导读:1建站知识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网站seo优化培训建设网站公司。

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 cas网seo优化趋势e --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 sub网站seo优化诊断工具ject 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网站seo优化培训列的子查询，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

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