if在sql里怎么用

select * from tb1 where 1=1 and  isnull(nullif(@a,''),id)=id

CREATE TABLE tb1 ( id INT, col1 VARCHAR(10) )
INSERT  tb1
        SELECT  1 ,
                'AAAAA'
        UNION ALL
        SELECT  2 ,
                'BBBBB'
        UNION ALL
        SELECT  3 ,
                'CCCCC'
        UNION ALL
        SELECT  4 ,
                'DDDDD'

DECLARE @a INT
SET @a = 3
IF @a IS NOT NULL
    OR @a <> '' 
    SELECT  *
    FROM    tb1
    WHERE   1 = 1
            AND id = @a

ELSE 
    SELECT  *
    FROM    tb1
    WHERE   1 = 1
DROP TABLE tb1

CREATE PROCEDURE [dbo].ParameterSniffTest    
(    
    @p_CustomerId int,
    @p_Status int,
    @p_FromDate datetime,
    @p_ToDate datetime
) 
AS   
BEGIN

    SET NOCOUNT ON  
    DECLARE
        @Parm               NVARCHAR(MAX),
        @sqlcommand         NVARCHAR(MAX) = N''

        SET @sqlcommand = 'SELECT * FROM ParameterSniffProblem WHERE 1=1'
        
　　　　 IF(@p_CustomerId IS NOT NULL)
            SET @sqlcommand = CONCAT(@sqlcommand,'AND CustomerId=@p_CustomerId ')

        IF(@p_Status IS NOT NULL)
            SET @sqlcommand = CONCAT(@sqlcommand,'AND OrederStatus=@p_Status ')

        IF(@p_FromDate IS NOT NULL)
            SET @sqlcommand = CONCAT(@sqlcommand,'AND CreateDate>=@p_FromDate ')

        IF(@p_ToDate IS NOT NULL)
            SET @sqlcommand = CONCAT(@sqlcommand,'AND CreateDate<=@p_ToDate ')

        
　　　　SET @Parm= '@p_CustomerId int,
                   @p_Status 　　int,
                   @p_FromDate 　datetime,
                   @p_ToDate 　　datetime '
        
　　　　EXEC sp_executesql @sqlcommand,@Parm,
                          @p_CustomerId    =    @p_CustomerId,
                          @p_Status        =    @p_Status,
                          @p_FromDate      =    @p_FromDate,
                          @p_ToDate        =    @p_ToDate 
END
GO

select * from tb1 where   id=@a OR ISNULL(@a,'')=''