Sum sales from different tables - mysql

Question

I have 5 tables related to sales. Three of them are like this:
Product_table_image
They are called product_a, product_b, product_c
The other tables are the time_id table which contains the reference for the date_id and the customers' table, which contains the details of customers.
time_id table
and
customer_table
The 3 tables refer to the sales of different products, but the products are not important in this context, because what I need is to sum up all the values per month per customer. There are cases when one or more customers might not have made a purchase of a certain product, which means not all customers ids will be in all products tables and that's what I can't figure out how to solve. It seems that my code is only able to fetch and sum when the clients have made purchases in all 3 tables.
So this is what I was able to come up with:
SELECT C.customer_id, ROUND((A.pa + B.pa + C.pc)* 1, 2) AS total,C.month_id
FROM (SELECT customer.customer_id, SUM(product_a.amount) AS pa , time_id.month_id FROM customer
INNER JOIN product_a on customer.customer_id = product_a.customer_id
INNER JOIN time_id on product_a.date_id = time_id.date_id
GROUP BY customer.customer_id, time_id.month_id) AS A
CROSS JOIN
(SELECT customer.customer_id, SUM(product_a.amount) AS pb , time_id.month_id FROM customer
INNER JOIN product_b on customer.customer_id = product_b.customer_id
INNER JOIN time_id on product_b.date_id = time_id.date_id
GROUP BY customer.customer_id, time_id.month_id) AS B
CROSS JOIN
(SELECT customer.customer_id, SUM(product_a.amount) AS pc , time_id.month_id FROM customer
INNER JOIN product_c on customer.customer_id = product_c.customer_id
INNER JOIN time_id on product_c.date_id = time_id.date_id
GROUP BY customer.customer_id, time_id.month_id) AS C
GROUP BY C.month_id, C.customer_id
ORDER BY C.month_id;
I've been stuck in it for a while, so any help is appreciated!

Answer 1

I have setup the tables and some sample data to make it more real.
create table month(id int, name varchar(20), primary key (id));
insert into month (id, name) values (1, 'January'),(2,'February'),(3,'March'),(4,'April');
create table year(id int, name varchar(4), primary key (id));
insert into year(id, name) values (2019, '2019'),(2020,'2020'),(2021,'2021');
create table time (id int, month_id int, year_id int, primary key (id));
alter table time add constraint fk_month FOREIGN KEY (month_id) REFERENCES month (id);
alter table time add constraint fk_year FOREIGN KEY (year_id) REFERENCES year (id);
insert into time (id, year_id, month_id) values
(1, 2019, 1),(2, 2019, 2),(3,2019,3),(4,2019,4),
(5, 2020, 1),(6, 2020, 2),(7,2020,3),(8,2020,4),
(9, 2021, 1),(10, 2021, 2),(11,2021,3),(12,2021,4);
create table customers (id int, name varchar(100), city varchar(100), country varchar(100), primary key (id));
insert into customers (id, name, city, country) values
(1, 'Google', 'San Francisco', 'US'),
(2, 'Ambev', 'São Paulo', 'BR'),
(3, 'Merck', 'Darmstadt', 'GE');
create table sales_of_product_a (id int, customer_id int, date_id int, amount decimal(10,2), primary key (id));
alter table sales_of_product_a add constraint fk_pa_time FOREIGN KEY (date_id) REFERENCES time (id);
-- only customer 1 - Google and 3 - Merck purchased product A
insert into sales_of_product_a (id, customer_id, date_id, amount) values
(1, 1, 1, 100.10),(2,1,2,200.20),(3,1,3,300.30),(4,1,4,400.40),
(5, 1, 5, 500.50),(6,1,6,600.60),(7,1,7,700.70),(8,1,8,800.80),
(9, 3, 1, 130.10),(10,3,2,230.20),(11,3,3,330.30),(12,3,4,430.40),
(13, 3, 5, 530.50),(14,3,6,630.60),(15,3,7,730.70),(16,3,8,830.80);
create table sales_of_product_b (id int, customer_id int, date_id int, amount decimal(10,2), primary key (id));
alter table sales_of_product_b add constraint fk_pb_time FOREIGN KEY (date_id) REFERENCES time (id);
-- only customer 1 - Google purchased product B
insert into sales_of_product_b (id, customer_id, date_id, amount) values
(1, 1, 1, 100.10),(2,1,2,200.20),(3,1,3,300.30),(4,1,4,400.40),
(5, 1, 5, 500.50),(6,1,6,600.60),(7,1,7,700.70),(8,1,8,800.80),
(9, 1, 9, 900.90),(10,1,10,1000.01),(11,1,11,1100.11),(12,1,12,1200.12);
create table sales_of_product_c (id int, customer_id int, date_id int, amount decimal(10,2), primary key (id));
alter table sales_of_product_c add constraint fk_pc_time FOREIGN KEY (date_id) REFERENCES time (id);
-- only customer 3 - Merck purchased product C
insert into sales_of_product_c (id, customer_id, date_id, amount) values
(1, 3, 1, 130.10),(2,3,2,230.20),(3,3,3,330.30),(4,3,4,430.40),
(5, 3, 5, 530.50),(6,3,6,630.60),(7,3,7,730.70),(8,3,8,830.80),
(9, 3, 9, 930.90),(10,3,10,1030.01),(11,3,11,1130.11),(12,3,12,1230.12);
The SQL you might be looking for would be something like.
with all_sales as (
select pa.customer_id, tt.month_id, sum(pa.amount) as amount from sales_of_product_a pa inner join time tt on (pa.date_id = tt.id) where tt.year_id = 2019 group by pa.customer_id, tt.month_id
union all
select pb.customer_id, tt.month_id, sum(pb.amount) as amount from sales_of_product_b pb inner join time tt on (pb.date_id = tt.id) where tt.year_id = 2019 group by pb.customer_id, tt.month_id
union all
select pc.customer_id, tt.month_id, sum(pc.amount) as amount from sales_of_product_c pc inner join time tt on (pc.date_id = tt.id) where tt.year_id = 2019 group by pc.customer_id, tt.month_id
),
sales_per_customer_per_month as (
-- summary of all sales of all products per customer per month
select customer_id, month_id, sum(amount) as amount from all_sales group by customer_id, month_id
),
customers_month as (
select c.id, c.name, c.city, c.country, m.id as month_id, m.name as month_name from customers c inner join month m on true
)
select c.id,c.name,c.city,c.country,c.month_id, coalesce(s.amount,0) as amount, sum(coalesce(s.amount,0)) over (partition by c.id order by c.id,c.month_id) as total
from customers_month c
left join sales_per_customer_per_month s on (s.customer_id = c.id and s.month_id = c.month_id)
order by c.id,c.month_id;
The result of query above is following.
The concepts used are linked below.
Window Functions:
https://dev.mysql.com/doc/refman/8.0/en/window-functions.html
Common Table Expressions (CTE):
https://www.mysqltutorial.org/mysql-cte/