Difference between NVL and NVL2

  NVL(exp1,exp2)
  if the exp1 is NULL then it will return exp2 otherwise it will return exp1.

  NVL2(exp1,exp2,exp3)
  if the exp1 is NULL then it will return exp3 otherwise exp2.

  here is another function which is used to check for null values

  COALESCE(exp1,exp2,expr3....)
  it will return first OT NULL expression or value.