SQL累加注意点

declare @test varchar(50)
select @test

declare @otherPrivateProducts varchar(max)
        set @otherPrivateProducts=''
        select @otherPrivateProducts=isnull(@otherPrivateProducts,'')+isnull(EproductIdStr,'') from EprivateShowRoom where EcompanyId={0}
        select @otherPrivateProducts;