Cartesian Or NOT

Question

Hello All,
When I run following query, I got 24 row.SELECT * FROM CLASSES, COURSES, REGISTRATIONS, USERS
WHERE (
(CLASSES.COURSE_ID = COURSES.COURSE_ID)
AND (REGISTRATIONS.CLASS_ID = CLASSES.CLASS_ID)
AND (REGISTRATIONS.USER_ID = USERS.USER_ID)
When I add 2 tables, I got 52 records. SELECT *
FROM CLASSES, COURSES, REGISTRATIONS, USERS, LOCATIONS_RF, LOCATIONS
WHERE ((CLASSES.COURSE_ID = COURSES.COURSE_ID)
AND (REGISTRATIONS.CLASS_ID = CLASSES.CLASS_ID)
AND (REGISTRATIONS.USER_ID = USERS.USER_ID)
and (CLASSES.CLASS_ID = LOCATIONS.CLASS_ID)
AND (LOCATIONS.LOC_ID = LOCATIONS_RF.LOC_ID )
Any help!
Thanks,
NY

Answer 1

If LOCATION is child of CLASSES then a single record of classes can be connected to two records of locations (say LOC1 and LOC2).
Each of these two records of location can be connected to one record of locations_rf, say LOC_RF1 and LOC_RF2.
Hence starting for a single record of classes you can get two distinct records of locations_rf, which one would you like to display?
1) Both of them in a single string? Then is a string aggregation problem there are many exaples on this forum, try to search
2) Both of them in different records? then the query is already ok
3) one of them based on a choise algorithm? Then let us know the algorithm
4) One of them random? You can write something like this:
SELECT col1, col2, col3, col4, min(locations_rf.somecolumn)
FROM CLASSES, COURSES, REGISTRATIONS, USERS, LOCATIONS_RF, LOCATIONS
WHERE ((CLASSES.COURSE_ID = COURSES.COURSE_ID)
AND (REGISTRATIONS.CLASS_ID = CLASSES.CLASS_ID)
AND (REGISTRATIONS.USER_ID = USERS.USER_ID)
and (CLASSES.CLASS_ID = LOCATIONS.CLASS_ID)
AND (LOCATIONS.LOC_ID = LOCATIONS_RF.LOC_ID )
group by col1, col2, col3, col4
)Max
http://oracleitalia.wordpress.com