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.

+------------         


        

Answer 1

Assuming each item_id, category_id is unique,

select *, count(item_id) as c from item_category
where category_id in (201, 202)
group by item_id
having c = 2;

Replace c = 2 with c = (count of categories)

Answer 2

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.

Answer 3

I don't have the time to provide the exact query right now, but try something like this:

select item_category.* from 
item_category ,  item_category ic1, item_category ic2 where 
ic1.category_id = 201 and 
ic2.category_id = 202 and 
ic1.item_id = ic2.item_id and 
item_category.item_id = ic1.item_id and 
item_category.item_id = ic2.item_id;

Maybe conditions are wrong but you can try something this way.

Answer 4

Use either a JOIN, or a GROUP BY:

SELECT category_id, fieldhere, anotherfield
FROM item_category
WHERE category_id in (201,202)
GROUP BY category_id
HAVING count(category_id) = 2

Answer 5

You could try:

SELECT * FROM item_category WHERE category_id IN (201, 202, ...)

(or, as the comments say... you may not.)