Join query with multiple criteria

Question

I have these tables

places(place_id, place_name)
places_criteria(place_id, criterion_id)
criteria(criterion_id, criterion_name)

"places_criteria" have foreign keys to "places" and "criteria". I can get many places by one criterion.

SELECT p.place_id, p.place_name
FROM places p INNER JOIN places_criteria pc ON p.place_id = pc.place_id
WHERE pc.criterion_id = < some_id >

How to get many places by multiple criteria?

For example : Disneyland is a place (place_id = 1), it is "good" (criterion_id = 1) and "interesting" (criterion_id = 2).

places_criteria's data:

place_id   criterion_id
1            1
1            2

Now I want to get the places which are "good" AND "interesting".

Answer 1

The hard coded version is to join the table on itself for each criterion you need...

SELECT
  c1.place_id
FROM
  places_criteria      AS c1
INNER JOIN
  places_criteria      AS c2
    ON c2.place_id = c1.place_id
WHERE
      c1.criterion_id = 1
  AND c2.criterion_id = 2

That is efficient, but requires dynamic SQL if you want to query a dynamic number of criterion_ids.

The alternative is to use IN (1,2) (or a join, etc, to perform a similar function) to get the answer to Places with criterion 1 <OR> 2 , then using a HAVING clause to include only places with two different criterion.

SELECT
  place_id
FROM
  places_criteria
WHERE
  criterion_id IN (1,2)
GROUP BY
  place_id
HAVING
  COUNT(DISTINCT criterion_id) = 2

Answer 2

You can do this :

SELECT p.place_id, p.place_name
FROM places p, places_criteria pc1, places_criteria pc2, places_criteria pc3
WHERE p.place_id = pc2.place_id and pc1.criterion_id = < some_id >
and  p.place_id = pc2.place_id and pc2.criterion_id = < some_id >
and p.place_id = pc3.place_id and pc3.criterion_id = < some_id >

But I have a doubt regarding my understanding of your question : why do you show us the criteria table ?

Answer 3

Use in to select from a set. You can use a subselect to get the right place ids, or you can use distinct as I do, which is probably somewhat faster in MySQL.

select distinct
  p.place_id,
  p.place
from
  places p
  inner join places_criteria pc on pc.place_id = p.place_id
  inner join criteria c on c.criterion_id = pc.criterion_id
where
  /* Either by name */
  c.criterion_name in ('crit A', 'crit B', 'crit C')
  /* Or by id */
  OR c.criterion_id in (1, 2, 3, 4, 5)

Solution 2. Match all criteria. Do this, by checking the count.

select
  p.place_id,
  p.place
from
  places p
  inner join places_criteria pc on pc.place_id = p.place_id
  inner join criteria c on c.criterion_id = pc.criterion_id
where
  c.criterion_name in ('crit A', 'crit B', 'crit C')
group by 
  p.place_id,
  p.place
having
  count(*) = 3 /* the number of criteria */

Answer 4

If you want all the places that meet one of several criteria:

SELECT p.place_id, p.place_name
FROM places p
INNER JOIN places_criteria pc ON p.place_id = pc.place_id
WHERE pc.criterion_id = < some_id >
OR pc.criterion_id = < some_id2 >
OR pc.criterion_id = < some_id3 >
GROUP BY p.place_id, p.place_name

If you want all the places that meet all of several criteria:

SELECT p.place_id, p.place_name
FROM places p
INNER JOIN places_criteria pc1 ON p.place_id = pc1.place_id
INNER JOIN places_criteria pc2 ON p.place_id = pc2.place_id
INNER JOIN places_criteria pc3 ON p.place_id = pc3.place_id
WHERE pc1.criterion_id = < some_id >
AND pc2.criterion_id = < some_id2 >
AND pc3.criterion_id = < some_id3 >