我有两个表student_behaviours（behaviour_id [PK]，admission_no，rule_id [FK]）和behaviour_rules（rule_id，behavior，points）

HOW to get sum(behaviour_rules.points). if behaviour_rules contain only two row with 1 and -1 point. and a admission_no get 5 1point and 2(-1) points then i expect return 3 points. but it always return 0.

SELECT SUM(behaviour_rules.points) 
FROM student_behaviours 
INNER JOIN behaviour_rules ON student_behaviours.rule_id = behaviour_rules.rule_id 
WHERE student_behaviours.admission_no = 2001 

In your ON clause You are joining the same table student_behaviours.rule_id = student_behaviours.rule_id you should joint the two tables tudent_behaviours.rule_id = behaviour_rules.rule_id

SELECT SUM(behaviour_rules.points) 
FROM student_behaviours 
INNER JOIN behaviour_rules ON student_behaviours.rule_id = behaviour_rules.rule_id 
WHERE student_behaviours.admission_no = 2001