在sqlserver 中用一条语句实现 如果为空显示全部数据

--生成测试数据
if not object_id('Tempdb..T_20081010') is null
    drop table T_20081010
Go
Create table T_20081010([ID] int,[S_ID] nvarchar(1),[P_ID] nvarchar(10))
Insert T_20081010
select 1,'a','2009-02-00' union all
select 2,'a','2009-02-28' union all
select 3,'b','2009-01-28' union all
select 4,'b','2009-01-25' union all
select 5,'b','2009-02-27' union all
select 6,'b','2009-02-26'
Go
--Select * from T_20081010 

--实现：
declare @P int 
set @P=''
select * from T_20081010
where (case  when (id=@p) then  1  when @p='' then 1  else 0 end)=1 

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