Limit multiple joined results to 1, by order - mysql

Question

I have persons, monsters and vampires, all of which can have multiple files referenced from a files table.
Simplified tables:
files: id, name;
character: id, type;
person: id, characterId FK character.id;
monster: id, characterId FK character.id;
vampire: id, characterId FK character.id;
person_files: personId FK person.id, fileId FK file.id, order;
monster_files: monsterId FK monster.id, fileId FK file.id, order;
vampire_files: vampireId FK vampire.id, fileId FK file.id, order;
As input I have a character.id. Am trying to get only one file.id while looking up the lowest ordered files in person_files, vampire_files and monster_files.
If person_files returns a result, we're done. Otherwise look up vampire_files and / or monster_files. Should return null if no files are found.
I could do something like
SELECT *
FROM character
LEFT JOIN person ON person.characterId = character.id
LEFT JOIN monster ON monster.characterId = monster.id
LEFT JOIN vampire ON vampire.characterId = monster.id
LEFT JOIN files as f1 ON person.fileID = f1.fileId
LEFT JOIN files as f2 ON monster.fileID = f2.fileId
LEFT JOIN files as f3 ON monster.fileID = f3.fileId
and filter out the data with where/min/max etc.
However it seems unnecessary to triple join on the files table when I am just looking up one file.id. Is there a subquery possible to prevent joining so many times and return just one myFileId? Would like to do this as efficiently as possible.
ie
LEFT JOIN (SELECT *one file.id*, MIN(order) as myFileId FROM person/monster/vampire...)

Answer 1

One optimization I could think of would be to select one fileId value based on which of person, monster or vampire table has a non-null field. The query will look like this:
SELECT * FROM (
SELECT COALESCE(person.file_id, monster.file_id, vampire.file_id) AS realFileId
FROM character
LEFT JOIN person ON person.characterId = character.id
LEFT JOIN monster ON monster.characterId = monster.id
LEFT JOIN vampire ON vampire.characterId = monster.id
) x
LEFT JOIN files f
ON x.realFileId=f.fileId
If you want to make life simpler for yourself, you may also consider adding a characterType field to character table and use it together with CASE operator to determine which table you need to join:
SELECT * FROM (
SELECT COALESCE(person.file_id, monster.file_id, vampire.file_id) AS realFileId
FROM character
LEFT JOIN person ON person.characterId = character.id
AND character.characterType="person"
LEFT JOIN monster ON monster.characterId = monster.id
AND character.characterType="monster"
LEFT JOIN vampire ON vampire.characterId = monster.id
AND character.characterType="vampire"
) x
LEFT JOIN files f
ON x.realFileId=f.fileId
While this looks more complex than the previous version, the optimizer in this case should optimize away the joins you won't be using (since you'll be literally joining on FALSE) and thus speed up the query quite a bit.