两列相减的问题

[dl_wang]

表中有两列，a b

select a-b as c from table1 where id='1234'

想在碰到一个问题，如果b为空（null）的话 上面的语句得到的c也为null，怎么解决呢？或者怎么能够让b为null时转换成0

[shiqiang_03]

SELECT NVL(A, 0) - NVL(B, 0) AS C FROM TABLE WHERE ID = '1234'

[ldw2545398]

select nvl(a,0)-nvl(b,0) as c from table1 where id='1234'

[hjianke]

select a-nvl(b,0) as c from table1 where id='1234'