समस्या 1:
SELECT tag_id, tag_name, count(*)
FROM (
SELECT tag_id, tag_name FROM tags
UNION ALL
SELECT tag_id, tag_name FROM more_tags
) s
GROUP BY tag_id, tag_name
समस्या 2:
SELECT tag_id, tag_name, 'not present in more tags' as description
FROM tags LEFT JOIN more_tags ON tags.tag_id=more_tags.tag_id
WHERE more_tags.tag_id IS NULL
UNION ALL
SELECT tag_id, tag_name, 'not present in tags' as description
FROM tags RIGHT JOIN more_tags ON tags.tag_id=more_tags.tag_id
WHERE tags.tag_id IS NULL
समस्या 3:
SELECT tag_id, tag_name, COUNT(*)
FROM tags INNER JOIN more_tags ON tags.tag_id=more_tags.tag_id
GROUP BY tag_id, tag_name
