Select all projects that have matching tags

Question

I\'m trying to find the most efficient way of dealing with this but I must tell you front-head I\'ve made a mess of it. Looked around SO and found nothing of relevance so he

Answer 1

In any of the following cases, if you don't know the PROJECT.num/PROJECT_TO_TAGS.project_id, you'll have to join to the PROJECTS table to get the id value for finding out what tags it has associated.

Using IN

SELECT p.*
  FROM PROJECTS p
  JOIN PROJECTS_TO_TAGS pt ON pt.project_id = p.num
 WHERE pt.tag_id IN (SELECT x.tag_id
                       FROM PROJECTS_TO_TAGS x
                      WHERE x.project_id = 4)

Using EXISTS

SELECT p.*
  FROM PROJECTS p
  JOIN PROJECTS_TO_TAGS pt ON pt.project_id = p.num
 WHERE EXISTS (SELECT NULL
                 FROM PROJECTS_TO_TAGS x
                WHERE x.project_id = 4
                  AND x.tag_id = pt.tag_id)

Using JOINS (this the most efficient one!)

The DISTINCT is necessary because JOINs risk duplicated data turning up in the resultset...

SELECT DISTINCT p.*
  FROM PROJECTS p
  JOIN PROJECTS_TO_TAGS pt ON pt.project_id = p.num
  JOIN PROJECTS_TO_TAGS x ON x.tag_id = pt.tag_id
                         AND x.project_id = 4