﻿ table 行转列的sql详解 - 鸿网互联

# table 行转列的sql详解

1 创建数据表
CREATE TABLE [dbo].[StuScore](
[stuid] [int] NOT NULL,
[subject] [nvarchar](30) NULL,
[score] [decimal](5, 1) NULL
)
2 插入测试数据
stuid subject score
3 chinese 76.0
3 math 73.0
4 chinese 82.0
5 chinese 66.0
5 math 93.0
6 chinese 67.0
7 math 83.0
8 chinese 77.0
8 math 84.0
3 行转列后的结果
stuid chinese math
3 76.0 73.0
4 82.0 0.0
5 66.0 93.0
6 67.0 0.0
7 0.0 83.0
8 77.0 84.0

1 行转列，一个重点就是怎么样知道有多少列，怎么样创建这些列？我们可以先把这个问题搁置，而假设这些列是已知的。 例如示例数据中，可以先假设subject的数据[chinese,math]是已知的，这样问题就简化了许多
2 当已知了chinese,math后，我们至少要先得到转换后的tabel结构

select stuid, 0 as chinese, 0 as math from dbo.StuScore

stuid chinese math
3 0 0
3 0 0
4 0 0
5 0 0
5 0 0
6 0 0
7 0 0
8 0 0
8 0 0
3 接着就需要往这个数据集中去填充chinese, math的数据
select stuid,
case subject when 'chinese' then score else 0 end as chinese,
case subject when 'math' then score else 0 end as math
from dbo.StuScore

stuid chinese math
3 76.0 0.0
3 0.0 73.0
4 82.0 0.0
5 66.0 0.0
5 0.0 93.0
6 67.0 0.0
7 0.0 83.0
8 77.0 0.0
8 0.0 84.0
4 细心的读者会发现步骤3中的结果与我们想要的已经非常接近了，只需再做一个sum()处理，就OK了
select stuid,
sum(case subject when 'chinese' then score else 0 end ) as chinese,
sum(case subject when 'math' then score else 0 end ) as math
from dbo.StuScore group by stuid

stuid chinese math
3 76.0 73.0
4 82.0 0.0
5 66.0 93.0
6 67.0 0.0
7 0.0 83.0
8 77.0 84.0

5 要获取subject的数据其实很简单
select distinct subject from dbo.StuScore

declare @sql varchar(2000)
set @sql=''
select @sql =@sql+ ',case subject when '''+subject+''' then 1 else 0 end as ' + subject
from (select distinct subject from dbo.StuScore) as sub
print @sql
message打印的信息如下：
,case subject when 'chinese' then 1 else 0 end as chinese,case subject when 'math' then 1 else 0 end as math
6 最后我们就需要将前面步骤综合起来，得到最终的sql
declare @sql varchar(2000)
set @sql='select stuid'
select @sql =@sql+ ',sum(case subject when '''+subject+''' then score else 0 end) as ' + subject
from (select distinct subject from dbo.StuScore) as sub
set @sql=@sql + ' from dbo.StuScore group by stuid'
exec(@sql)
stuid chinese math
3 76.0 73.0
4 82.0 0.0
5 66.0 93.0
6 67.0 0.0
7 0.0 83.0
8 77.0 84.0

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