select s1.id as id,s1.score as math,s2.score as english,s1.score+s2.score as sum 
from student as stu join score as s1 join score as s2 
where stu.id=s1.id AND s1.id=s2.id AND s1.subject='math' AND s2.subject='english' 
union 
select 'avg' as id,avg(s1.score) as math,avg(s2.score) as english,avg(s1.score+s2.score)
from score as s1 join score as s2