Equations in SQL (MySQL) - mysql

Question

I have a table with a cost_maintence column that has cost for the entire year(52) weeks. I also have a table of renters, and a table of renter_units where there is a week_owned column that has the week number the renter rented. I am trying to figure out how I could calculate the cost for each renter. The equation I came up with is:
what each person owes = (cost_maintence/52) * #weeks each renter
rented
Is there any way I could get the value from a query?
create table renters(
id,
lname,
fname,
phone_num);
create table unit(
id,
unit_name,
unit_num,
cost_maintence);
create table renters_unit(
renters_id,
unit_id,
week_owned);
This is the query I came up with but I have no way of testing it out
select r.lname, r.fname, count(ru.week_owned),
sum(u.cost_maintence/52*count(ru.week_owned))
from renters r, renters_units ru, units u
where r.id = ru.renter_id
and ru.unit_id = u.id
and u.unit_name =unitname
and u.unit_num = unitnum
group by lname
order by lname,fname asc;

Answer 1

Here's an example. The inner query will get you amount owed per item, and the outer query sums that to find the total owed per person.
SELECT fname, SUM(owes) AS total_due
FROM (
SELECT r.fname,
r.id,
u.unit_name,
u.cost_maintence/52*COUNT(ru.week_owned) AS owes
FROM renters AS r
INNER JOIN renters_unit AS ru ON r.id = ru.renters_id
INNER JOIN unit AS u ON u.id = ru.unit_id
GROUP BY r.id, u.id
) AS t
GROUP BY id
Try it out with a SQLFiddle demo
Example Schema:
create table renters(
id int,
lname varchar(20),
fname varchar(20),
phone_num varchar(20));
create table unit(
id int,
unit_name varchar(30),
unit_num int,
cost_maintence int);
create table renters_unit(
renters_id int,
unit_id int,
week_owned int);
INSERT INTO renters VALUES (1, 'Peterson', 'Chaz', '8675309');
INSERT INTO unit VALUES (1, 'Skateboard', 1337, 52);
INSERT INTO unit VALUES (2, 'Flamethrower', 5432, 104);
INSERT INTO renters_unit VALUES (1, 1, 1);
INSERT INTO renters_unit VALUES (1, 1, 2);
INSERT INTO renters_unit VALUES (1, 1, 4);
INSERT INTO renters_unit VALUES (1, 2, 4);
INSERT INTO renters_unit VALUES (1, 2, 5);
By this, we can see that Chaz should owe $7 for the year (had a skateboard for 3 weeks at $1 per week, and a flamethrower for 2 weeks at $2 per week).
The inner query gives the following:
FNAME UNIT_NAME OWES
Chaz Skateboard 3
Chaz Flamethrower 4
And the outer:
FNAME TOTAL_DUE
Chaz 7

Answer 2

SELECT t.renters_id, SUM(u.cost_maintence)/52
FROM unit u JOIN renters_unit t ON t.unit_id = u.id
GROUP BY t.renters_id