sql-server - 为什么是无限循环？

Question

以下代码将无限循环。然而，在SSMS中停止查询后，表为空。将 if @id is not null 更改为 if @@rowcount > 0 将获得预期结果。为什么当#t为空时@id没有得到空值？

select 100 id into #t
l:
declare @id int
select top 1 @id = Id from #t 

if @id is not null
begin
    print @id

    --begin try
        raiserror ('Test error', 16, 10)
    --end try
    --begin catch
    --end catch  
    delete from #t where Id = @id
    goto l
end

(1 row(s) affected)
100
Msg 50000, Level 16, State 10, Line 10
Test error

(1 row(s) affected) --------- The row was deleted here.
100
Msg 50000, Level 16, State 10, Line 10
Test error

(0 row(s) affected)
100
Msg 50000, Level 16, State 10, Line 10
Test error

......

更新:
将 select top 1 @id = Id from #t 更改为 set @id = (select top 1 Id from #t) 效果很好。

Answer 1

如果#t为空，此选择不会为@id分配值:

select top 1 @id = Id from #t 

修复它的一种方法:

set @id = null
select top 1 @id = Id from #t