nullif (A, B)
-->return null if A equals B (, else return A)
eg:
select nullif('','') --null
isnull(A, B)
--> if A is null, return B (; else return B)
eg:
select isnull(null,null) --null
select isnull(3,null) --3
select isnull(null,'44') --44
select isnull('33','44') --33