Mapping Classes to Relational Tables (Continued)

To instantiate a FullTime employee object, we only need to read from one table:

select name, dob, hiredate, ssn, 
  payrate from FullTime
  where name = 'Sarah Jackson'

Similarly, we can update an Exempt employee by writing to one table:

update Exempt set dob=
  "06/30/1962", set hiredate=
  "06/15/1997", set ssn=
  "567-89-0123", set salary = 
  52000.00
  where name = 'Nicky Colson'

Note that updating unchanged columns is not necessary, but will highlight some of the differences between horizontal/union mapping and vertical mapping. To remove an object:

delete from Manager where name = 
  'Susan Jekel'

In the next case, we need to perform the query on each of the horizontal tables and return the combined results to find employees hired since "01/01/1998":

select name, dob, hiredate, ssn, 
  payrate from PartTime
  where hiredate >= "01/01/1998"
select name, dob, hiredate, ssn, 
  payrate from FullTime
  where hiredate >= "01/01/1998"
select name, dob, hiredate, ssn, 
  salary from Exempt
  where hiredate >= "01/01/1998"
select name, dob, hiredate, ssn, 
  salary from Manager
  where hiredate >= "01/01/1998"

Part 2 of this article will take a look at union and vertical inheritance mapping and include SQL examples for creating, reading, updating, removing, and retrieving objects under those approaches.

About the Author
Richard Jensen is a senior architect at Persistence Software.