MySQL Select ID's which occur on different rows with multiple specific values for a column

Question

I\'m trying to select items from an associative table that have satisfy two or more values of the same field, sounds confusing, let me explain.

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Answer 1

Your expression in a WHERE clause works against a single row of the joined result set. That's why WHERE category_id = 201 AND category_id = 202 doesn't work -- because it can't be two values on a single row.

So you need some way to join two rows from the table into one row of the result set. You can do this with a self-join:

SELECT c1.item_id
FROM item_category AS c1
INNER JOIN item_category AS c2 ON c1.item_id = c2.item_id
WHERE c1.category_id = 201 AND c2.category_id = 202

This technique is hard to scale up when you want to search for three, four, five or more values, because it requires N-1 joins to match N values.

So another method is to use GROUP BY:

SELECT c.item_id, COUNT(*) AS cat_count
FROM item_category AS c
WHERE c.category_id IN (201,202)
GROUP BY c.item_id
HAVING cat_count = 2

Both techniques are okay, and work better in different circumstances.