折叠T-SQL?

我强烈反对这样做。这是应该在应用程序层中处理的事情。

但是如果必须:
Concatenating Row Values in Transact-SQL

亲爱的投票人:临时表和游标必须至少和上面接受的递归查询和自定义函数解决方案一样有效。克服对光标的恐惧,有时候它们是最有效的解决方案。有时,它们是唯一的解决办法。处理它。

请不要因为一些博主说“它不好”就放弃sql的每个构造的main;运用你自己的判断和一些常识。我尽可能避免使用游标,但不会达到解决方案不健壮的程度。

--initial data table
create table #tmp (
    id int,
    subid int,
    txt varchar(256)
)

--populate with sample data from original question
insert into #tmp (id,subid,txt) values (1, 1, 'Hello')
insert into #tmp (id,subid,txt) values (1, 2, 'World')
insert into #tmp (id,subid,txt) values (1, 3, '!')
insert into #tmp (id,subid,txt) values (2, 1, 'B')
insert into #tmp (id,subid,txt) values (2, 2, 'B')
insert into #tmp (id,subid,txt) values (2, 3, 'Q')

--temp table for grouping results
create table #tmpgrp (
    id int,
    txt varchar(4000)
)

--cursor for looping through data
declare cur cursor local for
    select id, subid, txt from #tmp order by id, subid

declare @id int
declare @subid int
declare @txt varchar(256)

declare @curid int
declare @curtxt varchar(4000)


open cur

fetch next from cur into @id, @subid, @txt

set @curid = @id
set @curtxt = ''

while @@FETCH_STATUS = 0 begin
    if @curid <> @id begin
        insert into #tmpgrp (id,txt) values (@curid,@curtxt)
        set @curid = @id
        set @curtxt = ''
    end
    set @curtxt = @curtxt + isnull(@txt,'')
    fetch next from cur into @id, @subid, @txt
end

insert into #tmpgrp (id,txt) values (@curid,@curtxt)

close cur

deallocate cur

--show output
select * from #tmpgrp

--drop temp tables
drop table #tmp
drop table #tmpgrp

declare  @tmp table (id int, subid int,txt varchar(256) )  
--populate with sample data from original question 
insert into @tmp (id,subid,txt) values (1, 1, 'Hello') 
insert into @tmp (id,subid,txt) values (1, 2, 'World') 
insert into @tmp (id,subid,txt) values (1, 3, '!') 
insert into @tmp (id,subid,txt) values (2, 1, 'B') 
insert into @tmp (id,subid,txt) values (2, 2, 'B') 
insert into @tmp (id,subid,txt) values (2, 3, 'Q') 

解决方案

Select id, fold = (Select cast(txt as varchar(100)) from @tmp t2 where t1.id = t2.id for xml path(''))
from @tmp t1
group by t1.id

将其包装在一个函数中,以便执行一次。。。

DECLARE @returnValue varchar(4000)

SELECT @returnValue = ISNULL(@returnValue + ', ' +  myTable.text, myTable.text)
FROM myTable 

RETURN @returnValue

对于少量记录,这将起作用。。。任何超过5或10的SQL函数都太多,需要按照其他人的建议将其移动到应用层。