MySQL Select ID's which occur on different rows with multiple specific values for a column
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.
+-----------------------+
| item_id | category_id |
+-----------------------+
| 1 | 200 |
| 1 | 201 |
| 1 | 202 |
| 2 | 201 |
| 2 | 202 |
| 3 | 202 |
| 3 | 203 |
| 4 | 201 |
| 4 | 207 |
+-----------------------+
In the table, I want to be a开发者_如何学Pythonble to select only items which are in the categories that I pass. So for example, if I pass category IDs of 201 and 202, I would want only items that are in BOTH categories (they can have other categories but need to be in at least the categories I'm querying), so in this case, I would want items 1 and 2 only since they are the only ones that are in categories 201 and 202.
My initial SQL statement was something like
SELECT * FROM item_category WHERE category_id = 201 AND category_id = 202;
But obviously that won't work.
SELECT * FROM item_category WHERE category_id = 201 OR category_id = 202;
The above query also won't work because it'll return items 4 and 3 as well.
So how would I go about only selecting items that have to at least be in both categories?
Keep in mind that I might be passing more than 2 category IDs.
Thank you for your help.
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.
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
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)
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.
You could try:
SELECT * FROM item_category WHERE category_id IN (201, 202, ...)
(or, as the comments say... you may not.)
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