计算相关表之间的总和

Question

I created a test database so I can illustrate my problem:

create table A(
    id int(11) primary key not null,
    price decimal(10,2)
);

create table B(
    id int(11) primary key not null,
    id_a int(11) not null,
    foreign key(id_a) references A(id)
    on update cascade
    on delete restrict
);

insert into A values
(1,25),
(2,30),
(3,35);

insert into B values
(1,1),
(2,1),
(3,2),
(4,2),
(5,3);

That is a simplified example of some articles(A) and their prices, and a bill(B) on which there is id of bill and foreign key that represent what article is bought.

I need query to find profit from all sold articles. So to go through table B and and find the sum of all prices of sold articles.

Answer 1

You could just join the two tables:

SELECT SUM(price)
FROM   a
JOIN   b ON b.id_a = a.id