SQL & PL/SQL

Hi,

I want to add a not null constraint to and exisitng table, but the table already has some null values in that column.

Emp

Emp_id  Name

1               axada

2              

3               sdkdd

Here the data is already present i.e 2 empid has Null as name. I have to add a not null constraint sot that new values will not null but i also dont want to change the alreadt exisitng data that has NULL value.