Retrieving records that has both tag T1 and T2

Question

Hi all,

I need an sql query that will retrieve all items that have both tags, not any of tags. I already have a query but returns all items that hove both tags, not expected result. Find detailed description below. Thanks!

Table structure:

ITEMS TABLE
------------
item_id
item_name

TAGS TABLE
----------
tag_id
tag_name

ITEMTAGS TABLE
---------------
tag_id
item_id

Query:

SELECT Items.* FROM Items 
INNER JOIN ItemTags ON Items.item_id = ItemTags.item_id
WHERE ItemTags.tag_id IN (T1, T2)
GROUP BY Items.item_id

Result: All items that have T1 or T2

Expected result: All items that have both T1 and T2

Answer 1

If your database supports the intersect keyword (SqlServer does) you can write:

SELECT Items.* 
FROM Items 
WHERE Items.item_id in 
/* intersection between items that have the tag T1 
   and the ones that have the tag T2 */ 
(
    SELECT item_id FROM ItemTags WHERE tag_id = T1
    INTERSECT 
    SELECT item_id FROM ItemTags WHERE tag_id = T2
)

If not, you will need to do something like:

SELECT Items.* 
FROM Items 
WHERE Items.item_id in 
(
    SELECT ItemTags1.item_id 
    FROM ItemTags as ItemTags1 
        INNER JOIN ItemTags as ItemTags2 
        ON ItemTags1.item_id = ItemTags2.item_id 
    WHERE 
            ItemTags1.tag_id = T1
        AND ItemTags2.tag_id = T2
)

On second query take care of possible performance problems because of ItemTags1.item_id = ItemTags2.item_id . You will need an index on the foreign key to work well.

Answer 2

select i.*
from items i, itemtags it1, itemtags it2
where i.item_id=it1.item_id and it1.tag_id=T1
and i.item_id=it2.item_id and it2.tag_id=T2;