的毗连结果在一个字符串中的存储过程

Question

在我的存储过程返回空值，我串联我的结果到像一个字符串：

Problem1|Problem2|Problem3 

当我试图在一个变量@display来存储，它将返回null 。当我运行查询而不将其存储到变量时，它会返回结果。不知道为什么它返回null？

declare @display nvarchar(500) 

select 
@display = @display + gp2.ProblemDesc + '|' 
from (SELECT gp.GrantProblemID, gp.ProblemDesc, gps.GrantProblemStatus, row_number() over (partition by gp.grantproblemid order by p.periodid desc) ranking from MergedProgressReport.dbo.[PeriodAgencyGrantProblems] pagp 
inner join MergedProgressReport.dbo.PeriodAgencyGrants pag on pag.PeriodAgencyGrantsID = pagp.PeriodAgencyGrantID and pag.GrantNumber = '2011UMWX0001' 
inner join MergedProgressReport.dbo.periodagencyinfo pai on pag.PeriodAgencyInfoID = pai.PeriodAgencyInfoID inner join MergedProgressReport.dbo.Periods p on p.PeriodID = pai.PeriodID 
inner join MergedProgressReport.dbo.GrantProblemStatus gps on pagp.GrantProblemStatusID = gps.GrantProblemStatusID inner join MergedProgressReport.dbo.GrantProblems gp on pagp.GrantProblemID = gp.GrantProblemID and gp.GrantProblemAreaID = 7) xx 
right join MergedProgressReport.dbo.GrantProblems gp2 on xx.GrantProblemID = gp2.GrantProblemID and xx.ranking = 1 where gp2.GrantProblemAreaID = 7 

Select @display 

Answer 1

试试这个。可能你需要初始化变量。

declare @display nvarchar(500) 
set @display = '' 

总之，在声明变量之后编写查询之前，您已经缺少set语句。