NVL (a,b)

　　--当a=null时，返回b,否则返回a

NVL2 (a, b, c)

　　-- 当a=null时，返回c，否则返回b

NULLIF (expr1, expr2)

　　--当a=b时，返回null，否则返回a

COALESCE (expr1, expr2, ..., exprn)

　　--从左至右找到第一个不为null的值

SQL> select comm, nvl(comm,0) from emp;      COMM NVL(COMM,0)--------- -----------                    0   300.00         300   500.00         500                    0  1400.00        1400                    0                    0                    0     0.00           0                    0                    0                    0                    0 13 rows selectedSQL> select comm, nvl2(comm,111,000) from emp;      COMM NVL2(COMM,111,000)--------- ------------------                           0   300.00                111   500.00                111                           0  1400.00                111                           0                           0                           0     0.00                111                           0                           0                           0                           0 13 rows selected SQL> select nullif('abc','abc'),nullif('abc','123') from dual; NULLIF('ABC','ABC') NULLIF('ABC','123')------------------- -------------------                    abc SQL> select coalesce(null,null,1,null,2,null) from dual; COALESCE(NULL,NULL,1,NULL,2,NU------------------------------                             1