Question

I have a tag table and three tableA,tableB,tableC tables that are taggable along with three tableA_tag, tableB_tag and tableC_tag tables to join them.

My application allows to remove tags from tableA, tableB or tableC and I want to keep only the tags currently used in one of them.
So far, I’ve wrote the following trigger:

CREATE TRIGGER plop after DELETE
ON tableA_tag
FOR EACH row
  DELETE FROM tag
  WHERE  tag.id = old.tag_id
         AND (SELECT Sum(countTags.c) AS s
              FROM   (SELECT Count(*) AS c
                      FROM   tableC_tag
                      WHERE  tag_id = old.tag_id
                      UNION
                      SELECT Count(*) AS c
                      FROM   tableB_tag
                      WHERE  tag_id = old.tag_id) countTags) = 0; 

/* Same trigger for table tableB_tag  */

/* And same again for table tableC_tag */

But it doesn’t work because it checks if the tag is still used in tableC_tag or tableB_tag but does not check in tableA_tag and can so delete tags even if they are still in use. And I cannot check tableA_tag because it’s the same table that fired the trigger.

So, how can I do ?

Answer 1

Hope this works…

CREATE TRIGGER plop after DELETE

ON tableA_tag

FOR EACH row

  DELETE FROM tag

  WHERE  tag.id = old.tag_id 

    and not exists(select ' ' from tableC_tag where tableC_tag.tag_id =tag.id)
    and not exists(select ' ' from tableB_tag where tableB_tag.tag_id =tag.id)   
    and not exists(select ' ' from tableA_tag where tableA_tag.tag_id =tag.id);