Relevance in Fulltext Search

Create Fulltext index first:
CREATE FULLTEXT INDEX indx_text
ON attachment (text)

Add relevance field for select:
SELECT MATCH('text') AGAINST ('financial') as Relevance FROM attachment WHERE MATCH
('text ') AGAINST('+financial +bank' IN
BOOLEAN MODE) HAVING Relevance > 0.2 ORDER
BY Relevance DESC

full search documentation