使用TRIGGER在mySQL INSERT上的计时问题

Question

For an iPad app I have created a web service which creates a new table row in my teams table when the user starts the app. The PHP generates a unique ID and fills some other fields as well.

Additionally I have created a trigger on the teams table in mySQL on insert where a teamnumber is generated automatically depending on the already inserted rows for that specific day, project and group.

Unfortunately I seem to have a timing issue when inserting new rows to my table sometimes. If two apps create a team in the same second (milisecond?) the result of the mySQL trigger will be the same teamnumber for both apps. So instead of having teamnumber 1 and 2 both apps have teamnumber 1.

My teamtable looks something like this:

TeamID | pProject | group | teamnumber | languagecode | created_at

And the trigger for mySQL:

CREATE TRIGGER after_team_insert BEFORE INSERT ON teams 

FOR EACH ROW
BEGIN

SELECT COUNT( * ) INTO @counter
FROM teams
WHERE DATE( created_at ) = DATE( NOW( ) )
AND teams.group = NEW.group
AND teams.pProject = NEW.pProject;

SET NEW.teamnumber = CAST(@counter AS UNSIGNED) + 1;

END;

I think my problem has something to do with parallel threads that insert rows into the database and therefor get a wrong count when the trigger is working.

Does anyone have a solution for this problem or do I have to use some kind of queue in PHP to prevent this doubling of teamnumbers?

Thanks in advance, Carsten.

Answer 1

I guess you can use a lock table ! http://dev.mysql.com/doc/refman/5.0/fr/lock-tables.html

Answer 2

If, for any reason, you can't lock your table you can try to handle the problem differently:

• Add a unique constraint on the columns group / pProject / teamnumber.
• Handle any constraint error in your code, check if the teamnumber is the problem, and then try to reevaluate it.
• Ensure you're teamnumber has been set. If not, try again.

Not that clean, a bit tricky, but it should do the work if you don't mind team number 3 was inserted after team number 4.