The function DECODE compares its first argument to 
one or more search expressions, which are paired with result expressions. 

Any search or result expression can be null. 

If a search is successful, the corresponding result is returned. 

In the following example, if the column rating is null, DECODE returns the value 1000:


SELECT DECODE(rating, NULL, 1000, 'C', 2000, 'B', 4000, 'A', 5000)
   INTO credit_limit FROM accts WHERE acctno = my_acctno;