按多个值过滤一对多关系

Question

This has got to be a common thing, but I'm not having luck Googling around.

I have a table of categories:

Table A
catid | text
1     | Category A
2     | Category B
3     | Category C

This table is then joined to a map that keeps track of what categories a photo is in

Table B
catid | photoid
1     | 1
2     | 1
1     | 2
3     | 3

What I need to do is filter the query so that I return only the photos that are in EVERY category selected. For example:

1. If the user selects categories A and B, I return photo 1.
2. If the user selects just category A, I return photos 1 and 2.
3. If the user selects categories A,B and C - I return nothing.

Thanks for any help you can give.

-Matt

Answer 1

One of the easiest ways to do it is:

SELECT 
    * 
FROM 
    B 
WHERE 
    B.catid IN (1, 2) 
GROUP BY 
    B.photoid 
HAVING 
    COUNT(B.photoid) = 2

To match 3 categories you would do:

SELECT 
    * 
FROM 
    B 
WHERE 
    B.catid IN (1, 2, 3) 
GROUP BY 
    B.photoid 
HAVING 
    COUNT(B.photoid) = 3

You can also join the same table multiple times. Or do a sub-query. I would test a couple of different methods to see which executes most quickly for your dataset.