MYSQL Optimising JOINED UNION SELECT query - mysql

Question

I've re-written this query a few times now (it's Monday) with the attempt of finding the most efficient way of getting the data I require however I'm not sure I'm even approaching it correctly at the moment.
To summarise the problem;
Users have two sets of tags (key_terms, project_terms), there's a link table between each of these between users and tags tables.
I would like to pull out any users that have specified tags in either table. Ideally it'd also include the 'most relevant' tag to that user - but lets put that aside for now.
users
| id | name |
| 1 | dayjo |
| 2 | stackoverflow |
tags
| id | tag |
| 1 | tag1 |
| 2 | tag2 |
user_key_term
| user_id | tag_id |
| 1 | 1 |
| 1 | 2 |
| 2 | 1 |
project_key_term
| user_id | tag_id |
| 1 | 3 |
| 2 | 3 |
What I want to be able to query on is the named tags, i.e if I search for "tag1" both users should be returned, however if I search for "tag2" only User 1 should be returned.
My Solutions
I tried by selecting users, and joining tags twice (one for each link table), this seemed to work ok but wasn't sure it was the best way, and couldn't figure out how to get the most relevant tag.
SELECT t1.tag, t2.tag as most_relevant_tag, users.* FROM users
LEFT JOIN user_key_term ON user_key_term.user_id = users.id
LEFT JOIN tags t1 ON user_key_term.tag_id = t1.id
LEFT JOIN project_key_term ON project_key_term.user_id = users.id
LEFT JOIN tags t2 ON project_key_term.tag_id = t2.id
WHERE t1.tag IN ('tag1','tag2') OR t2.tag IN ('tag1','tag2')
GROUP BY users.id;
My next attempt was a UNION select, but this one feels dirty;
SELECT users.* FROM
`users`
INNER JOIN (
SELECT project_key_term.user_id, tags.id, tags.tag FROM project_key_term
JOIN tags ON tags.id = project_key_term.tag_id AND tags.tag IN ('tag1')
UNION ALL
SELECT user_key_term.user_id,tags.id, tags.tag FROM user_key_term
JOIN tags ON tags.id = user_key_term.tag_id AND tags.tag IN ('tag1')
) tags ON tags.user_id = users.id
WHERE tags.tag IN ('tag1')
GROUP BY users.id;
But
I've tried running EXPLAIN on both queries to see which is best, but it doesn't reveal anything particularly useful to me. Especially because at the moment there's not a lot of data in the tables, there will potentially be hundreds / thousands of tags.
Any help on the 'correct' or best practice way to do this sort of query would be great!

Answer 1

The union query can be simplified to:
SELECT users.*
FROM users
INNER JOIN (
SELECT user_id,tag_id
FROM project_key_term
UNION ALL
SELECT user_id,tag_id
FROM user_key_term
) alltags ON alltags.user_id = users.id
INNER JOIN tags t on t.id = alltags.tag_id
where t.tag IN ('tag1')
Edit: Getting the most relevant tags
SELECT score, t.tag, users.*
FROM users
INNER JOIN (select user_id, tag_id, count(*) as score
from (SELECT user_id,tag_id
FROM project_key_term
UNION ALL
SELECT user_id,tag_id
FROM user_key_term
) alltags
group by user_id,tag_id) tagcounts ON tagcounts.user_id = users.id
INNER JOIN tags t on t.id = tagcounts.tag_id
where t.tag IN ('tag1','tag2','tag3')
ORDER BY score DESC