Въз основа на отговора в връзката, която сте публикували, можете да направите нещо подобно
SELECT id,SUM(relevance) as total_relevance FROM (
SELECT
id,
(MATCH(title) AGAINST ('search string')) AS relevance
FROM Cards
UNION
SELECT
Cards.id,
(MATCH(brigade) AGAINST ('search string')) AS relevance
FROM Brigades
INNER JOIN CardBrigades ON Brigades.id=brigade_id
INNER JOIN Cards ON card_id=Cards.id
UNION
SELECT
Cards.id,
(MATCH(identifier) AGAINST ('search string')) AS relevance
FROM Identifier
INNER JOIN CardIdentifier ON Identifier.id=identifier_id
INNER JOIN Cards on card_id=Cards.id
) AS combined_search
GROUP BY id
HAVING total_relevance > 0
Не съм сигурен колко добре би се представило това. Може би е по-добре да разгледате друго решение като Solr, Lucene или дори NoSQL машина за съхранение.