MySQL: Select all rows with the same ID, if a column from a joined table contains x

Question

(Giving an extremely simplified sample here, for the sake of clarity.)

I have a many-to-many relationship in a database (MySQL with Doctrine2/Symfony2 entities, but i'd like to know the answer in plain SQL), with a simple 2-column "join table" in between:

item
id itemname
1 Cat
2 Mouse

tag
id tagname
1 Predator
2 Prey
3 Likes Cheese
4 Flying

item_tag
item_id tag_id
1 1
1 3
2 2
2 3

The standard join query for this:

SELECT itemname, tagname FROM item
JOIN item_tag ON  item_tag.item_id = item.id
JOIN tag ON item_tag.tag_id = tag.id

gives me all the data i'm interested in, with certain rows being double of course:

item
itemname tagname
Cat Predator
Cat Likes Cheese
Mouse Prey
Mouse Likes Cheese

Now I need to filter this with a simple WHERE statement, so I add something like this:

WHERE tagname = 'prey'

But that only returns a single row of course. I need to get all rows for all items which have a 'prey tag' — so all the rows for the mouse item. The fact that the mouse likes cheese is a pretty important detail to know when you are searching for prey!

The point is of course to enter a search query for a certain tag name (without knowing the item names in advance), but once it returns a set of items, I would like to see all the other tags as well for the items in that set.

Is this possible with one query, or would I be forced to re-query to grab the remaining associated tags?

Answer 1

You can get the item_id from each prey like this:

SELECT it.item_id
FROM item_tag it
JOIN tag t ON t.id = it.tag_id AND t.tagname = 'prey';

Then, you can use that as a subquery in the `WHERE clause of your original query to get all rows for those items:

SELECT i.itemname, t.tagname
FROM item i
JOIN item_tag it ON it.item_id = i.id
JOIN tag t ON t.id = it.tag_id
WHERE i.id IN(
   SELECT it.item_id
   FROM item_tag it
   JOIN tag t ON t.id = it.tag_id AND t.tagname = 'prey');

Here is an SQL Fiddle example.

Answer 2

Thought I'd also include my Symfony2 / Doctrine solution, based on McAdam331's answer for SQL.

I used a second query builder instance in the item repository for the subquery. You just need to use different indexes than used in the main query builder, of course.

$qb = $this->createQueryBuilder('i');
$sub = $this->createQueryBuilder('subi');

$search = $qb->expr()->orX(
    $qb->expr()->like('i.name', ':s'),
    $qb->expr()->in('i.id', 
        $sub->select('subi.id')
        ->join('subi.tags', 'subt')
        ->where($sub->expr()->like('subt.tagname',':s'))->getDQL()
    ),
    ... // various other search expressions are included in the OR clause
)

$qb->select('i','t')->join->('i.tags','t') // etcetera
   ->where($search)
   ->setParameter('s', "%".$searchString."%")