MySQL 8.0

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create table tablea
(id int, val int)
insert into tablea (id, val) values (1,10),(2,15),(3,20)
create table tableb
(id int, val int)
insert into tableb (id, val) values (1,4),(2,5),(3,6),(4,7)
create table tableab
(id1 int, id2 int)
insert into tableab (id1, id2) values (1,1),(1,2),(2,1),(3,2),(3,4)
select x.id,x.val,sum(tableb.val)+x.val as vals from 
(select * from tablea inner join tableab on tablea.id=tableab.id1) x inner join
tableb on x.id2=tableb.id 
group by x.id,x.val,id1


id
val
vals


1
10
19

2
15
19

3
20
32