为什么MySQL对列匹配会返回否定结果？

Question

I just found the great benefit of MySQL match against but just stumbled upon trouble.

If the joined table2 has no paired rows. MySQL match against returns a negative value for the paired/join column that has NULL as value. Removing the column from the match group returns a positive value for relevance. Why would one NULL column cause the whole match to return -2?

select *, match(t1.column1, t1.column2, t1.column3, t1.column4, t2.column5) against ('search string' in boolean mode) as relevance
from table1 t1
left join table2 t2 on (t2.id = t2.column_id)
/*having relevance > 0*/
order by relevance desc;

Answer 1

I guess.

What if search texts contain operator '-' or '+'?

relevance can be negative just when you have operator '-' on your search text