Question

I’ll try to explain my case as good as i can. I’m making a website where you can find topics by browsing their tags. Nothing strange there. I’m having tricky time with some of the queries though. They might be easy for you, my mind is pretty messed up from doing alot of work :P.

I have the tables ‘topics’ and ‘tags’. They are joined using the table tags_topics which contains topic_id and tag_id. When the user wants to find a topic they might first select one tag to filter by, and then add another one to the filter. Then i make a query for fetching all topics that has both of the selected tags. They might also have other tags, but they MUST have those tags chosen to filter by. The amount of tags to filter by differs, but we always have a list of user-selected tags to filter by. This was mostly answered in Filtering from join-table and i went for the multiple joins-solution.

Now I need to fetch the tags that the user can filter by. So if we already have a defined filter of 2 tags, I need to fetch all tags but those in the filter that is associated to topics that includes all the tags in the filter. This might sound wierd, so i’ll give a practical example 😛

Let’s say we have three topics: tennis, gym and golf.

• tennis has tags: sport, ball, court and racket
• gym has tags: sport, training and muscles

• golf has tags: sport, ball, stick and outside

  1. User selects tag sport, so we show all three tennis, gym and golf, and we show ball, court, racket, training, muscles, stick and outside as other possible filters.
  2. User now adds ball to the filter. Filter is now sport and ball, so we show the topics tennis and golf, with court, racket, stick and outside as additional possible filters.
  3. User now adds court to the filter, so we show tennis and racket as an additional possible filter.

I hope I’m making some sense. By the way, I’m using MySQL.

Answer 1

SELECT DISTINCT `tags`.`tag` FROM `tags` LEFT JOIN `tags_topics` ON `tags`.`id` = `tags_topics`.`tag_id` LEFT JOIN `topics` ON `tags_topics`.`topic_id` = `topics`.`id` LEFT JOIN `tags_topics` AS `tt1` ON `tt1`.`topic_id` = `topics`.`id` LEFT JOIN `tags` AS `t1` ON `t1`.`id` = `tt1`.`tag_id` LEFT JOIN `tags_topics` AS `tt2` ON `tt2`.`topic_id` = `topics`.`id` LEFT JOIN `tags` AS `t2` ON `t2`.`id` = `tt2`.`tag_id` LEFT JOIN `tags_topics` AS `tt3` ON `tt3`.`topic_id` = `topics`.`id` LEFT JOIN `tags` AS `t3` ON `t3`.`id` = `tt3`.`tag_id` WHERE `t1`.`tag` = 'tag1' AND `t2`.`tag` = 'tag2' AND `t3`.`tag` = 'tag3' AND `tags`.`tag` NOT IN ('tag1', 'tag2', 'tag3')