Question

Sorry for the abysmal title – if someone wants to change it for something more self-explanatory, great – I’m not sure how to express the problem. Which is:

I have a table like so:

POST_ID (INT)   TAG_NAME (VARCHAR)

    1              'tag1'
    1              'tag2'
    1              'tag3'
    2              'tag2'
    2              'tag4'
   ....

What I want to do is count the number of POSTs which have both tag1 AND tag2.
I’ve messed about with GROUP BY and DISTINCT and COUNT but I can’t construct a query which does the trick.
Any suggestions?

Edit: In pseudo sql, the query I want is:

SELECT DISTINCT(POST_ID) WHICH HAS TAG_NAME = 'tag1' AND TAG_NAME = 'tag2'; 

Thanks

Answer 1

Edit: because ‘TABLE’ was a poor choice for a missing tablename, I’ll suppose your table is called Posts.

Join the table against itself:

SELECT * FROM Posts P1
JOIN Posts P2
ON P1.POST_ID = P2.POST_ID
WHERE P1.TAG_NAME = 'tag1'
AND P2.TAG_NAME = 'tag2'