mysql中是否有机制可以基于任意值限制INSERT

Question

If I want to limit number of entries per user in this table:

CREATE TABLE `connections` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `user_id` int(11) unsigned NOT NULL,
  `word_id` int(11) unsigned NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `No duplicates` (`user_id`,`word_id`),
  KEY `word_id` (`word_id`),
  CONSTRAINT `connections_ibfk_1` FOREIGN KEY (`user_id`) REFERENCES `users` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `connections_ibfk_2` FOREIGN KEY (`word_id`) REFERENCES `words_en` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

And this limit can change and I store it in another table called settings.

Should I do first SELECT and than do php compare and than INSERT. or is there a mechanism to do this kind of operations directly in INSERT query?

Answer 1

DELIMITER $$
create
  trigger `some trigger` AFTER INSERT
  on table
  for each row begin
  if (select count(id) from someothertable)>=100 then
    delete from table  WHERE id = NEW.id -99;  
end$$
DELIMITER ;

I`m afraid I`m out of my depth,I think I could do it given enough time for research,but you`d better ask somebody more informed.Here is my half-assed trigger,not tested.

There is also Create Event in mysql you could use. http://www.sitepoint.com/how-to-create-mysql-events/