Looks simple, but I'm stumped!

Question

This should be so simple, but I'm drawing a blank. I have two tables.

Table:article_tag_pvt
colum: article_id
column: tag_id

Table: tag
column: tag_id
column: name

The article_tag_pvt table is a many-to-many pivot table.

The problem is, I need a query which given a list of tag names, will retrieves article Ids which ONLY match those tag names. The basic joining of these tables looks like this:

SELECT article_id
FROM article_tag_pvt pvt
INNER JOIN tag t ON t.tag_id = pvt.tag_id

I've already got a query which will retrieve article ids which match ANY tag names specified. This looks like this:

SELECT article_id
FROM article_tag_pvt pvt
INNER JOIN tag t ON t.tag_id = pvt.tag_id
WHERE t.name IN ('events','news')

I've tried these but no joy:

SELECT article_id
FROM article_tag_pvt pvt
INNER JOIN tag t ON t.tag_id = pvt.tag_id
WHERE t.name = 'events' AND t.name = 'news'

SELECT article_id
FROM article_tag_pvt pvt
INNER JOIN (
SELECT tag_id
FROM tag
WHERE name IN ('events','news')
) AS t
ON t.tag_id = pvt.tag_id

Any help would be greatly appreciated

Dave

Answer 1

My preference is for:

SELECT pvt.article_id
  FROM article_tag_pvt pvt
  JOIN tag t ON t.tag_id = pvt.tag_id AND t.name = 'events'
  JOIN tag t2 ON t2.tag_id = pvt.tag_id AND t2.name = 'news'

Alternative using GROUP BY/HAVING:

  SELECT pvt.article_id
    FROM article_tag_pvt pvt
    JOIN tag t ON t.tag_id = pvt.tag_id
   WHERE t.name IN ('events', 'news')
GROUP BY pvt.article_id, t.name
  HAVING COUNT(DISTINCT t.name) = 2

The COUNT(DISTINCT t.name) needs to equal the number of options you define for the t.name IN clause.

Answer 2

The problem is that you get ALL rows which have either event or news. If you never have more than 1 event or news in many to many table you can use GROUP BY and HAVING to solve it

SELECT article_id
FROM article_tag_pvt pvt
INNER JOIN tag t ON t.tag_id = pvt.tag_id
WHERE t.name = 'events' OR t.name = 'news' 
GROUP BY article_id HAVING count(*) = 2