As a side note, FKs do enforce other table specific properties like indexes on the fields being constrained, so they do add value there as well. But there's of course an extra cost on updates and inserts to see if the FK is violated. Waynn On 12/11/08, Colin Guthrie <gmane@xxxxxxxxxxxxxx> wrote: > 'Twas brillig, and Chris at 12/12/08 01:20 did gyre and gimble: >> Micah Gersten wrote: >>> Colin Guthrie wrote: >>>> The ON DELETE CASCADE option is key here... "DELETE FROM students >>>> where student_id=1" will remove all traces of that student from the >>>> db... all the course they've attended, all the instructors who have >>>> taught them etc. keeps things nice and tidy without having to put the >>>> structure in your code all over the place. >>>> >>>> Col >>>> >>> Why would you want to delete the instructors when deleting the student? >> >> I think he meant the link between the student & instructor (in the >> student_instructor table), not the instructor itself. > > lol, indeed, that's what I meant... Sorry I thought it was implied in > the context! > > Say you have the following layouts > > instructors: instructor_id, name > students: student_id, name > instructor_students: instructor_id, student_id > > > This structure would hold a list of instructors and a list of studends > and also a one to many mapping of instructors to students. > > If you delete a student the FK can cascade to the instructor_students > table and thus delete the records that indicate a given instructor (or > instructors) taught them. > > Col > > > -- > > Colin Guthrie > gmane(at)colin.guthr.ie > http://colin.guthr.ie/ > > Day Job: > Tribalogic Limited [http://www.tribalogic.net/] > Open Source: > Mandriva Linux Contributor [http://www.mandriva.com/] > PulseAudio Hacker [http://www.pulseaudio.org/] > Trac Hacker [http://trac.edgewall.org/] > > > -- > PHP General Mailing List (http://www.php.net/) > To unsubscribe, visit: http://www.php.net/unsub.php > > -- PHP General Mailing List (http://www.php.net/) To unsubscribe, visit: http://www.php.net/unsub.php
