Detta kan vara lika illa som O(n^2), men det fungerar:
create table QuestionTags (questionid int, tag int);
select q1.questionid, q2.questionid, count(*) as commontags
from QuestionTags q1 join QuestionTags q2
where q1.tag = q2.tag and q1.questionid < q2.questionid
group by q1.questionid, q2.questionid order by commontags desc;