Sum columns from two tables in sql - mysql

Question

I have two tables, one is the cost table and the other is the payment table, the cost table contains the cost of product with the product name.
Cost Table
id | cost | name
1 | 100 | A
2 | 200 | B
3 | 200 | A
Payment Table
pid | amount | costID
1 | 10 | 1
2 | 20 | 1
3 | 30 | 2
4 | 50 | 1
Now I have to sum the total of cost by the same name values, and as well sum the total amount of payments by the costID, like the query below
totalTable
name | sum(cost) | sum(amount) |
A | 300 | 80 |
B | 200 | 30 |
However I have been working my way around this using the query below but I think I am doing it very wrong.
SELECT
b.name,
b.sum(cost),
a.sum(amount)
FROM
`Payment Table` a
LEFT JOIN
`Cost Table` b
ON
b.id=a.costID
GROUP by b.name,a.costID
I would be grateful if somebody would help me with my queries or better still an idea as to how to go about it. Thank you

Answer 1

This should work:
select t2.name, sum(t2.cost), coalesce(sum(t1.amount), 0) as amount
from (
select id, name, sum(cost) as cost
from `Cost`
group by id, name
) t2
left join (
select costID, sum(amount) as amount
from `Payment`
group by CostID
) t1 on t2.id = t1.costID
group by t2.name
SQLFiddle

Answer 2

You need do the calculation in separated query and then join them together.
First one is straight forward.
Second one you need to get the name asociated to that payment based in the cost_id
SQL Fiddle Demo
SELECT C.`name`, C.`sum_cost`, COALESCE(P.`sum_amount`,0 ) as `sum_amount`
FROM (
SELECT `name`, SUM(`cost`) as `sum_cost`
FROM `Cost`
GROUP BY `name`
) C
LEFT JOIN (
SELECT `Cost`.`name`, SUM(`Payment`.`amount`) as `sum_amount`
FROM `Payment`
JOIN `Cost`
ON `Payment`.`costID` = `Cost`.`id`
GROUP BY `Cost`.`name`
) P
ON C.`name` = P.`name`
OUTPUT
| name | sum_cost | sum_amount |
|------|----------|------------|
| A | 300 | 80 |
| B | 200 | 30 |

Answer 3

A couple of issues. For one thing, the column references should be qualified, not the aggregate functions.
This is invalid:
table_alias.SUM(column_name)
Should be:
SUM(table_alias.column_name)
This query should return the first two columns you are looking for:
SELECT c.name AS `name`
, SUM(c.cost) AS `sum(cost)`
FROM `Cost Table` c
GROUP BY c.name
ORDER BY c.name
When you introduce a join to another table, like Product Table, where costid is not UNIQUE, you have the potential to produce a (partial) Cartesian product.
To see what that looks like, to see what's happening, remove the GROUP BY and the aggregate SUM() functions, and take a look at the detail rows returned by a query with the join operation.
SELECT c.id AS `c.id`
, c.cost AS `c.cost`
, c.name AS `c.name`
, p.pid AS `p.pid`
, p.amount AS `p.amount`
, p.costid AS `p.costid`
FROM `Cost Table` c
LEFT
JOIN `Payment Table` p
ON p.costid = c.id
ORDER BY c.id, p.pid
That's going to return:
c.id | c.cost | c.name | p.pid | p.amount | p.costid
1 | 100 | A | 1 | 10 | 1
1 | 100 | A | 2 | 20 | 1
1 | 100 | A | 4 | 50 | 1
2 | 200 | B | 3 | 30 | 2
3 | 200 | A | NULL | NULL | NULL
Notice that we are getting three copies of the id=1 row from Cost Table.
So, if we modified that query, adding a GROUP BY c.name, and wrapping c.cost in a SUM() aggregate, we're going to get an inflated value for total cost.
To avoid that, we can aggregate the amount from the Payment Table, so we get only one row for each costid. Then when we do the join operation, we won't be producing duplicate copies of rows from Cost.
Here's a query to aggregate the total amount from the Payment Table, so we get a single row for each costid.
SELECT p.costid
, SUM(p.amount) AS tot_amount
FROM `Payment Table` p
GROUP BY p.costid
ORDER BY p.costid
That would return:
costid | tot_amount
1 | 80
2 | 30
We can use the results from that query as if it were a table, by making that query an "inline view". In this example, we assign an alias of v to the query results. (In the MySQL venacular, an "inline view" is called a "derived table".)
SELECT c.name AS `name`
, SUM(c.cost) AS `sum_cost`
, IFNULL(SUM(v.tot_amount),0) AS `sum_amount`
FROM `Cost Table` c
LEFT
JOIN ( -- inline view to return total amount by costid
SELECT p.costid
, SUM(p.amount) AS tot_amount
FROM `Payment Table` p
GROUP BY p.costid
ORDER BY p.costid
) v
ON v.costid = c.id
GROUP BY c.name
ORDER BY c.name