Select count(*) into flag from employee where employee_id=100

If flag=1 then 

Update employee 

Else

Insert employee

End if;
 

MERGE INTO employee a
  USING employee b 
    ON ( b.employee_id=a.employee_id and b.employee_id=100)
  WHEN MATCHED THEN
    UPDATE SET a.last_name = 'abc' 
    
  WHEN NOT MATCHED THEN
    INSERT (last_name ,employee_id,email,job_id )
    VALUES ('xxxxx' ,100,'xxx','AD_PRES')

MERGE INTO employee a
  USING dual b 
    ON (a.employee_id=100)
  WHEN MATCHED THEN
    UPDATE SET a.last_name = 'abc' 
  WHEN NOT MATCHED THEN
    INSERT (last_name ,employee_id,email,job_id )
    VALUES ('xxxxx' ,100,'xxx','AD_PRES')