SQL & PL/SQL

Hi,

I am writing a trigger TR_EMP on a table EMP which has columns EMP_ID, EMP_NAME, ALT_EMP_ID.

Now I am updating ALT_EMP_ID for an EMP_ID(PK) which is unique.

If ALT_EMP_ID is null
then
      :new.ALT_EMP_ID = l_alt_emp_id;
end if;

As this ALT_EMP_ID is unique, same ID shouldn't be inserted again here. When data being inserted with 2 different sessions for 2 different EMP_ID there is a possible chance of inserting same ALT_EMP_ID for both which results in Unique error. I need to handle this exception. DUP_VAL_ON_INDEX or OTHERS Execption not able to handle this.

Can you help me out on how to handle such exception?