Oracle PL SQL

Nvl just replace null with value you given... however Coalesce will check the first occurrence of null value and replace with next value
Eg : Nvl(Emp_dob, '01-01-1990') if emp_dob is null it will just replace it with 01-01-1990

In coalesce(emp_dob,dob,'01-01-1990') if emp_dob is null then it will check dob and if that is also null then it will default it to 01-01-1990.

Hope this clears you the difference.