通过Case When语句作行列转换 echo

在DT

现在写出解决方案：

问题描述：

有一张表，数据如下:

/*
    Date        Tag
    2010-01-01   是
    2010-01-01   是
    2010-01-01   是
    2010-01-02   否
    2010-01-02   否
*/

要求转换结果：

/*
转换结果:

    Date        是    否
    2010-01-01    3    0
    2010-01-02    0    2

*/

用Case When语句解决：

select TheDate,SUM(case Tag when '是' then 1 else 0 end) [是],
               SUM(case Tag when '否' then 1 else 0 end) [否]
               from #Test
               group by TheDate

以上SQL语句，可以通过构造变量了实现：

create table #Test
(
    TheDate    varchar(30),
    Tag    varchar(10)
)

go

insert into #Test values ('2010-01-01','是');
insert into #Test values ('2010-01-01','是');
insert into #Test values ('2010-01-01','是');
insert into #Test values ('2010-01-02','否');
insert into #Test values ('2010-01-02','否');

select * from #Test;

--drop table #Test;


declare @tempSelect varchar(4000);
set @tempSelect = 'select TheDate';

select @tempSelect += ',SUM(case Tag when '''+ Tag +''' then 1 else 0 end) ['+ Tag +']'
    from (select distinct Tag from #Test) as tab;--重点在这句话select distinct Tag from #Test
    
select @tempSelect += ' ' + 'from #Test group by TheDate;'

print @tempSelect
exec(@tempSelect)

执行结果：

KO了，理解了。但我对于‘养起’还是不理解，算了，理解不了。