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Creating a Trigger


ThePsion5
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Hi again,My database consists of 3 tables, two of which contain pure data (Clients, Keywords) and the third (ClientsToKeywords) that is a relationship mapping between the two. I want to record the number of clients per keyword and the number of keywords per client and determined that the most efficient way to do this would be by creating a trigger that updates the value every time the ClientsToKeywords table is modified, but to be honest I'm new to triggers and I'm not sure how to do this. Unfortunately, I havn't been able to find a good online reference for triggers. What would the appropriate syntax be to do this, and where might I be able to find a good trigger tutorial?Database SchemaClients(ClientID, ClientName, ClientWebsite, NumKeywords)Keywords(KeywordID, Keyword, NumClients)ClientsToKeywords(ClientToKeywordID, ClientID, KeywordID, Rating)Note: Underlined fields are primary keys, bold fields are foriegn keys.

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what database are you using??? SQL Server? Oracle? MySql?

I'm using MySQL version 5.0.18...and I think I have a partial solution to my problem. If I understand the syntax correctly, this trigger should increment the NumKeywords field if a new entry is added to the ClientsToKeywords table or if a current entry is modified:
CREATE TRIGGER Increment_Num_Keywords AFTER UPDATE OF ClientID ON ClientsToKeywordsFOR EACH ROWWHEN new.ClientID = Clients.ClientIDUPDATE Clients SET  NumKeywords = NumKeywords + 1

I believe this should take care of incrementing the number of Keywords, but I still don't know how to decrement the old one if a client-keyword mapping is modified.

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Sorry to double-post.I think I may have found an easier solution to implement, although more computationally expensive for the Database.

CREATE TRIGGER Recount_Num_Keywords AFTER UPDATE OF ClientID ON ClientstoKeywordsBEGINUPDATE Clients SET NumKeywords (SELECT COUNT(*) FROM ClientsToKeywords WHERE ClientID = new.ClientIDEND

I believe that this would work properly...my main question is would this trigger function when values are inserted, deleted, and updated? Or only when values are modified using the UPDATE command?

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Is there a simple way to specify for this trigger to execute on UPDATE, INSERT, and DELETE?

I believe you can just list the three types one right after the other meaning:CREATE TRIGGER Recount_Num_Keywords AFTER UPDATE, INSERT, DELETE OF ClientID ON ClientstoKeywordsBEGINUPDATE Clients SET NumKeywords (SELECT COUNT(*) FROM ClientsToKeywords WHERE ClientID = new.ClientIDENDI am not positive though, so you might just give it a try. This was based off of some old documentation I read for Oracle.
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I believe you can just list the three types one right after the other meaning:CREATE TRIGGER Recount_Num_Keywords AFTER UPDATE, INSERT, DELETE OF ClientID ON ClientstoKeywordsBEGINUPDATE Clients SET NumKeywords (SELECT COUNT(*) FROM ClientsToKeywords WHERE ClientID = new.ClientIDENDI am not positive though, so you might just give it a try. This was based off of some old documentation I read for Oracle.

AFAIK that should work!
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Ok, I think I have it figured out. Here are the triggers I plan on using:

CREATE TRIGGER Increment_After_Insertion AFTER INSERT ON ClientsToKeywordsFOR EACH ROW BEGINUPDATE Clients SET NumKeywords = NumKeywords+1 WHERE ClientID = NEW.ClientID;UPDATE Keywords SET NumClients = NumClients+1 WHERE keywordID = NEW.KeywordID;END;CREATE TRIGGER Decrement_After_Deletion AFTER DELETE ON ClientsToKeywordsFOR EACH ROW BEGINUPDATE Clients SET NumKeywords = NumKeywords-1 WHERE ClientID = OLD.ClientID;UPDATE Keywords SET NumClients = NumClients-1 WHERE KeywordID = OLD.KeywordID;END;CREATE TRIGGER Modify_After_Update AFTER UPDATE ON ClientsToKeywordsFOR EACH ROW BEGINUPDATE Clients SET NumKeywords = NumKeywords+1 WHERE ClientID = NEW.ClientID;UPDATE Keywords SET NumClients = NumClients+1 WHERE keywordID = NEW.KeywordID;UPDATE Clients SET NumKeywords = NumKeywords-1 WHERE ClientID = OLD.ClientID;UPDATE Keywords SET NumClients = NumClients-1 WHERE KeywordID = OLD.KeywordID;END;

The Increment_After_Insertion trigger is activated when a row is inserted into the ClientsToKeywords table and increments the count when it finds a match in the Client table, and does the same for Keywords.The Decrement_After_Deletion trigger is activated when a row is removed from the ClientsToKeywords table and decrements the count when it finds the client who's info was removed.The Modify_After_Update does both since one client gained a keyword and another lost one, and does the same for keywords.Does this make sense? (Unfortunately, I couldn't combine the UPDATE, DELETE, and INSERT values in trigger syntax.)

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