Question about a conditional join

Question

Is it possible to do a conditional join based on a field in a table?
say I have the query below and the table fields ate the bottom. I am trying to get the exact syntax for doing a conditional join. Can any one help?
Thanks
select e.fname,e.lname,a.areaname
FROM employee e
JOIN area a ON (If e.newareaid is not null and date being searched for >= e.movedate then a.areaid = e.newareaid ELSE a.areaid = e.areaid
======================================================
For example
Employee table
empid
areaid
fname
lname
newareaid
movedate
Area table
areaid
areaname

Answer 1

Hi,
In this particular case, the complicated join condition can be simplified considerably.
There are 2 situations that have different join conditions, but in each situation a.areaid has to match something (let's call it join_id) from the employee table, and that can be determined just by looking at the row in the employee table. If we decide what join_id is before starting the join, then the join condition will be quite simple:
WITH     got_join_id
     SELECT     fname, lname, areaid, newareaid
     ,     CASE
              WHEN  newareaid              IS NOT NULL
              AND       date_being_searched_for   >= movedate
                  THEN  newareaid
               ELSE  areaid
          END     AS join_id
     FROM     employee
SELECT  e.fname, e.lname, a.areaname
FROM      got_join_id  e
JOIN      area           a   ON  a.areaid = e.join_id
;You could also skip the sub-query, and put the CASE expression in the join condition:
SELECT  e.fname, e.lname, a.areaname
FROM      got_join_id  e
JOIN      area           a   ON  a.areaid = CASE
                             WHEN  newareaid              IS NOT NULL
                                 AND       date_being_searched_for   >= movedate
                                    THEN  newareaid
                              ELSE  areaid
                         END
;This is basically what Hoek did, except Hoek complicated it by putting the comparison to a.areaid inside the CASE expression.
Edited by: Frank Kulash on Apr 15, 2013 3:22 PM