nvl(n,expr),如果n=null,返回expr的值,否则返回nSQL> select nvl(null,'is null') from dual;NVL(NUL
-------
is nullSQL> select nvl('123','is null') from dual;NVL
---
123
-------
is nullSQL> select nvl('123','is null') from dual;NVL
---
123
为空则返回'0',否则为自身.
select nvl(null,'is null') from dual;返回is null
select nvl('1243','is null') from dual;返回1243