sql 中case when then else end 的使用

select * from sysobjects where [xtype]='u'
go

if exists(select id from sysobjects where name='studentscore')
drop table studentscore--删除与实验冲突的表
go
create table studentscore--创建实验表
(
[id] int identity(1,1),
[name] nvarchar(20) not null,
subject nvarchar(20) not null,
score int not null
)
go

select * from studentscore
go

--添加实验数据
insert studentscore values ('张三','语文','60');
insert studentscore values ('张三','数学','65');
insert studentscore values ('张三','外语','70');
insert studentscore values ('李四','语文','80');
insert studentscore values ('李四','数学','90');
insert studentscore values ('李四','外语','85');
insert studentscore values ('王五','语文','70');
insert studentscore values ('王五','数学','71');
insert studentscore values ('王五','外语','75');
insert studentscore values ('赵六','语文','64');
insert studentscore values ('赵六','数学','67');
insert studentscore values ('赵六','外语','76');
go
select * from studentscore
go

我们先利用case when then else end 语句将行转为列:

select [name],语文=case when subject='语文' then score else 0 end from studentscore group by [name],subject,score

这里为了好理解只取一列,得到下面的结果

有了语文成绩行专列的例子后,我们很容易将其他两列也添加进来,

select [name],
语文=case 
when subject='语文' then score else 0
end,
数学=case 
when subject='数学' then score else 0
end,
外语=case 
when subject='外语' then score else 0
end
from studentscore
group by [name],subject,score

下面是查询后的结果:

现在只要把name相同的行合并到一起就ok了,

select [name],
语文=max(case 
when subject='语文' then score else 0
end),
数学=max(case 
when subject='数学' then score else 0
end),
外语=max(case 
when subject='外语' then score else 0
end)
from studentscore
group by [name]

好了,看看结果吧.

 

上面是列数可枚举时的代码,很多情况下到底有多少列是动态的或者不可知的.

这个时候需要拼下SQL语句了.

declare @sql varchar(8000)

set @sql = 'select [name],'
select @sql = @sql + 'sum(case subject when '''+subject+''' 
then score else 0 end) as '''+subject+''','
from (select distinct subject from studentscore) as a 
select @sql = left(@sql,len(@sql)-1) + ' from studentscore group by [name]'
exec(@sql)