create procedure GetRecord
@id int output,              --输出p_id和p_path
@path nvarchar(255) output
as

select top 1 @id = p_id, @path = p_path from n_project where p_flag = '0';
if(@id > 0)  
   Update n_project set p_flag = '1' where p_id = @id
else
  begin
    set @id = 0;      --若没有结果则给个默认值，否则直接返回NULL会使程序错误
    set @path = ' ';  --若p_path为NULL，则它也会返回NULL，从而造成程序错误
  end
if(@path is NULL)
begin
   set @path = ' ';
end

if @@error=0
print 'Good'
else
print 'Fail'
go

--测试程序
declare @idd int
declare @ppath nvarchar(255)
EXEC dbo.GetRecord @idd output,@ppath output
select '1'=@idd, '2'=@ppath
go

SQL SERVER中，按CTR+0，即可输入空值NULL

http://blog.csdn.net/gzq400/archive/2007/02/02/1501235.aspx