Question

I store some data in MySQL and want to filter out rows that match a given criteria.

Sounds easy, but it is not since there are some join criteria involved.

I do have the following tables:

items : id, ...
genres: id, name:varchar, item_id

Each item has multiple genres.

The query should filter out items if at least one genre does match a given genre name (or a set of names).

For example:

Item with id 1 has 3 genres
  - genre name = 'foo'
  - genre name = 'bar'
  - genre name = 'baz'

Item 1 may not be part of the result set if the given genre name is ‘bar’, [‘bar’, ‘baz’, ‘xyz’], etc.

I tried to left join the genres on the items and applied a WHERE statement with “genres.name NOT IN (?)”. ? is the given set of genre names.

This (of course) only works for items with exactly one genre. The same could be achieved by multiple WHERE conditions: WHERE name <> ‘a’ AND name <> ‘b’ …

Any ideas how to get this query done properly?

Thanks in advance!

Answer 1

You can use a correlated subquery.
Example:

SELECT id 
FROM   items i
WHERE  NOT EXISTS (SELECT 1 
                   FROM   genres g
                   WHERE  g.`name` = 'bar' 
                   AND    i.id = g.item_id);

A correlated subquery differs from other subqueries in that it references the outer query.
Correlated subqueries are run for each row projected/selected by the outer query.