RE: Use a rule or a transaction
От | Madel, Kurt |
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Тема | RE: Use a rule or a transaction |
Дата | |
Msg-id | C6F9B91B745CD4119F1500A0C9DD60C02E3490@exchhq01.usinspect.com обсуждение исходный текст |
Ответ на | Use a rule or a transaction ("Madel, Kurt" <KMadel@USInspect.com>) |
Ответы |
Re: Use a rule or a transaction
(Antoine Reid <antoiner@hansonpublications.com>)
|
Список | pgsql-sql |
Hey Antoine, I am using 7.0.2, and I would be in your debt if you created a working example. Basically, the structure of the query I would like to do is as such (there are six modules in the module table for each quarter: a_q1,b_q1,c_q1,d_q1,e_q1,morn_q1,a_q2.....). A student comes to register and selects all of the classes that are available in a given module, and then hits submit. Information will be inserted into the modules table as follows (in PHP): <?php .... $conn=pg_connect(blah, blah, blah)or die('Unable to connect to database'); pg_exec($conn,"insert into module (student_id,year) values ('$student_id','$year')") or die('Unable to insert record, please go back to registration page'); pg_exec($conn,"begin work"); pg_exec($conn,"update module set a_q1='$a_q1 where student_id=$student_id and year=$year"); pg_exec($conn,"update class set class.size=class.size+1 where class.id=$a_q1 and class.size<class.maxsize"); pg_exec($conn,"commit work"); //and so on for the other 23 module/quarters ... ?> If you could give me a working example of a trigger/function that would simplify this, that would be fantastic. Thanks, Kurt -----Original Message----- From: Antoine Reid [mailto:antoiner@hansonpublications.com] Sent: Tuesday, August 15, 2000 3:04 PM To: Madel, Kurt Cc: pgsql-sql@postgresql.org Subject: Re: [SQL] Use a rule or a transaction On Tue, Aug 15, 2000 at 02:52:24PM -0400, Madel, Kurt wrote: > Hello Antoine, > > I thought that triggers only worked for one table at a time. In this case, > I want the class.size incremented and checked for maxsize when I perform an > update or insert on the module table. I was under the impression that a > trigger would only work if I was performing an insert or update on the same > table that I wanted the trigger to act on. > > Please let me know if I am way off in regards to triggers. > > thanks, > Kurt > Hi! Well, the trigger, basically is a function that will be executed when you act on one table. If I understand you correctly, you have a table that is a list of classes. for each class, you want to have a 'reference count' (current usage) and a maximum. Trying to take your own words, 'module' is a list of items that are to be put member of 'classes'. classes is the table that has the maximum and current usage numbers. Assuming this is the case, what you do is following: (pseudo code.. not actual sql statements!) on insert on modules, update classes set current_usage=current_usage + 1 where the class_id is equal to the id of the row you just added in modules. on delete on modules, update classes set current_usage=current_usage - 1 ... on update on modules, IF-and-only-if the class_id changed, increment the new class, and decrement the old class. This all takes care of keeping the refcount in classes up to date. Now, for the maximum.. since your triggers on modules will always do an update on classes, we can create triggers on update on classes to make sure the maximum is not over.. create another trigger; create a trigger AFTER update on classes. abort the transaction with a proper message if class.current_usage > class.maximum. abort the transaction with proper message if class.current_usage < 0 (this shouldn't happen!!!!!) I have implemented ref counts in the past, I could probably come up with a working example really quick, if you want. I suspect the maximum check shouldn't be too hard either.. Oh BTW, this should work on 7.0.2, probably on 7.0; I really don't know about 6.x.x.. hope this helps antoine -- o Antoine Reid o> Alcohol and calculus <o> <|> antoiner@hansonpublications.com <| don't mix. Never drink |>\ antoiner@edmarketing.com >\ and derive. /<
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