Getting related tags from SQL Server when you have filtered down

Question

I asked this question before and got a great working answer.

what is the query to get "related tags" like in stack overflow

but i realized that SOF ac

Answer 1

Let's say you have a table called entity_tags linking entities with the tags like this :

entity_id : INTEGER
tag_id    : INTEGER

Let's say you user as selected N tags @1, @2, ... @N. To get the other tags associated with the results (and as a bonus, their frequency of occurence):

SELECT et.tag_id, COUNT(et.entity_id) as frequency FROM
 entity_tags AS et
 JOIN entity_tags AS et1 ON (et1.entity_id=et.entity_id AND et1.tag_id=@1)
 ...
 JOIN entity_tags AS etN ON (etN.entity_id=et.entity_id AND etN.tag_id=@N)
WHERE et.tag_id NOT IN (@1, @2, ... @N)
GROUP BY et.tag_id;

Answer 2

 SELECT t.tagName
 FROM tags t INNER JOIN tagsBridge tb on t.id=tb.tagID

 where tb.linkID in (

   select tb.linkID from tagsBridge tb
   where tb.tagID= @first_tag

   intersect

   select tb.linkID from tagsBridge tb
   where tb.tagID= @second_tag

   intersect

   select tb.linkID from tagsBridge tb
   where tb.tagID= @third_tag

   intersect

   select tb.linkID from tagsBridge tb
   where tb.tagID= @fourth_tag

   --- // you can continue
 )

)

Answer 3

I'd use something like this as the first version. @FirstTagID and @SecondTagID must be sorted by ID. You can query on more tags by adding more joins to the inner query.

SELECT tagName
FROM tags
WHERE id IN (
    SELECT tagID
    FROM tagsBridge
    WHERE linkID IN (
        SELECT t1.linkID
        FROM
            tagsBridge t1
            JOIN tagsBridge t2 ON t2.linkID=t1.linkID AND t2.tagID > t1.tagID
        WHERE t1.tagID=@FirstTagID AND t2.tagID=@SecondTagID
    )
    AND tagID!=@FirstTagID AND tagID!=@SecondTagID
)

Or if you want to include the number of links:

SELECT t.tagName, count(*)
FROM 
    tags t
    JOIN tagsBridge tb ON tb.tagID = t.id
WHERE
    tb.linkID IN (
        SELECT t1.linkID
        FROM
            tagsBridge t1
            JOIN tagsBridge t2 ON t2.linkID=t1.linkID AND t2.tagID > t1.tagID
        WHERE t1.tagID=@FirstTagID AND t2.tagID=@SecondTagID
    )
    AND t.tagID!=@FirstTagID AND t.tagID!=@SecondTagID
GROUP BY t.tagName

The same for three tags:

SELECT t.tagName, count(*)
FROM 
    tags t
    JOIN tagsBridge tb ON tb.tagID = t.id
WHERE
    tb.linkID IN (
        SELECT t1.linkID
        FROM
            tagsBridge t1
            JOIN tagsBridge t2 ON t2.linkID=t1.linkID AND t2.tagID > t1.tagID
            JOIN tagsBridge t3 ON t3.linkID=t2.linkID AND t3.tagID > t2.tagID
        WHERE t1.tagID=@FirstTagID AND t2.tagID=@SecondTagID AND t3.tagID=@ThirdTagID
    )
    AND t.tagID!=@FirstTagID AND t.tagID!=@SecondTagID AND t.tagID!=@ThirdTagID
GROUP BY t.tagName