Using "exception join" in SQL

SQL's "exception join" syntax is rarely understood and even more rarely used.

Even for the experienced iSeries guru, SQL's "exception join" syntax is rarely understood and even more rarely used.

Ironically, it's very similar to the (much more globally understood) "unmatched records with primary file" option in Query/400.

Consider the following two tables:

 
> select * from employee

EMPLOYEEID   EMPLOYEENAME                     DEPARTMENTID 
         1   Greg Graffin                                1 
         2   Dino Cazares                                2 
         3   Burton Bell                                 2 
         4   Lee Ving                                    4 
         5   Mike Levine                                 4 
         6   Devin Townsend                              6 
         7   Otep Shamaya                                7

> select * from department

DEPARTMENTID   DEPARTMENTNAME
           1     HR
           2     IS
           3     QA
           4     CR

To find employees with an invalid department:

> select
    Employee.employeeName
  from
    Employee
      exception join
    Department
      on
    Employee.departmentID = Department.departmentID

EMPLOYEENAME  
Devin Townsend
Otep Shamaya

To find departments that contain no employees:

>  select                                           
    Department.departmentName,
    Department.departmentID
  from
    Department                                     
      exception join                               
    Employee                                       
      on                                           
    Employee.departmentID = Department.departmentID

DEPARTMENTNAME   DEPARTMENTID
  QA                        3

The exception join is functionally equivalent to:

SELECT
  Employee.employeeName
FROM
  Employee
where
  not exists
    (select * from Department
     where Employee.departmentID = Department.departmentID)
or

>  select
    Employee.employeeName
  from
    Employee
  where
    Employee.departmentID not in
       (select departmentID from Department) 

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This was first published in June 2003

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