Nullif

The NullIf statement compares one value with another. If they match, it returns null, otherwise it returns the first. In this example, the NullIf, and the following Case statement get the same results.

Column Employee_Id  Heading Employee|Id
Column NullIf_JobId Heading NullIf|(E_Job_Id)
Column Case_JobID   Heading Case|(E_Job_Id)
Column Equals       Heading '='
Column E_Job_Id     Heading E.|Job_id
Column J_Job_Id     Heading J.|Job_id

Select E.Employee_Id, 
       E.Job_Id As E_Job_Id, 
       --------------------------------------------
                     Case When E.Job_Id = J.Job_Id 
                          Then '='
                          Else ' '
                      End 
                     As 
       Equals,
       --------------------------------------------
       J.Job_Id As J_Job_Id, 
       --------------------------------------------
                     Nullif(E.Job_Id, J.Job_Id) /* this is the same as the following case */
                     As
       NullIf_JobId,
       --------------------------------------------
                     Case When E.Job_Id = J.Job_Id   /* this is the same as the nullif */
                          Then Null 
                          Else E.Job_ID 
                      End 
                     As 
       Case_JobID
       --------------------------------------------
  From Employees E, 
       Job_History J
 Where E.Employee_Id = J.Employee_Id
 Order By E.Employee_Id;