Q: Explain the concept of NULL

In SQL NULL means the value is unknown. This is not same as 0 or the empty string ''. To check if the value in a column is NULL we use the clause "IS NULL"
Select * from emp where sal IS NULL; This statement will return all records with salary as null. However the statement Select * from emp where sal = NULL will not return any records.

Function nvl  is used to replace the null value with some other value.