MySQL Delete Row在需要时不删除多对多表中的值

Question

I am implementing my own blogging system to better understand the object oriented side of PHP, mySQL, and jQuery AJAX. I have a many-to-many relationship between Post_T and Tag_T, thus including an associative entity PostTag_T. When creating a post, I added functions that would check if the tag already exists, and either add the new tag and mapping, or just add the mapping to the existing tag.

The mishap that I noticed is that when deleting a Post, the tag row in Tag_T still exists while the mapping in PostTag_T is removed (set to ON DELETE CASCADE). How can I go about deleting the tag, if it isn't being used by any other post? I have the post ID while deleting the Post.

Useful Logic
Using the post id from deletion, delete from tag_t where in posttag_t the pID = post id AND no other posts (pID) in posttag_t is using the same tag id .

delete from tag_t where tID = (select tID from posttag_t where pID='post id') AND..

For your sanity, here is a visual representation

Created Post row in Post_T:

Mapping between post and tag in PostTag_T:

Created Tag row in Tag_T:

When Deleting: using AJAX POST with data that calls a php file in url (guessing this is where I might need to add a query that searches tables..)

Answer 1

DELETE FROM `tag_t` WHERE 
`tID` IN (SELECT `tID` FROM `posttag_t` WHERE `pID` = 'post_id') AND 
`tID` NOT IN (SELECT `tID` FROM `posttag_t` WHERE `pID` != 'post_id')

The last part of above query check is there any post using this tag or not.

Answer 2

When you delete a post due to ON DELETE CASCADE you no longer have rows in posttag_t table thus it's useless to try to match against it. That means that you left with only option to delete all tags that are not associated with any posts.

You can do that with a query

DELETE t
  FROM tag_t t
 WHERE NOT EXISTS
(
  SELECT *
    FROM posttag_t 
   WHERE tid = t.tid
);

Here is SQLFiddle demo

---

Now you can let a trigger to take care of that

CREATE TRIGGER gt_ad_posttag_t
AFTER DELETE ON post_t
FOR EACH ROW
  DELETE t
    FROM tag_t t
   WHERE NOT EXISTS
  (
    SELECT *
      FROM posttag_t 
     WHERE tid = t.tid
  );

Here is SQLFiddle demo