MySQL variable as result of subquery with elements of other table - mysql

Question

I have a working query:
SELECT count(*),
AVG(
(SELECT sum(order_fills.quant * order_fills.price)
FROM order_fills
WHERE `type`='closed'
AND order_fills.transaction_id = transactions.id) -
(SELECT sum(order_fills.quant * order_fills.price)
FROM order_fills
WHERE `type`='opened'
AND order_fills.transaction_id = transactions.id)) AS avgProfit,
SUM(
(SELECT sum(order_fills.quant * order_fills.price)
FROM order_fills
WHERE `type`='closed'
AND order_fills.transaction_id = transactions.id)) AS volume,
FROM transactions
WHERE transactions.status='closed';
to make the code more readable I wish to set some parts of that query into variables.
I wish to achieve something like that (it does not work, returns null):
SET #transSells =
(SELECT sum(order_fills.quant * order_fills.price)
FROM order_fills
WHERE type='closed'
AND order_fills.transaction_id = transactions.id);
SET #transBuys =
(SELECT sum(order_fills.quant * order_fills.price)
FROM order_fills
WHERE type='opened'
AND order_fills.transaction_id = transactions.id);
SELECT avg(#transSells - #transBuys) AS `avgProfit`,
sum(#transSells + #transBuys) AS `volume`
FROM transactions WHERE transactions.status='closed'
I think the problem might be that part of subquery WHERE statement is the result of the main query and another table (transactions.id):
AND order_fills.transaction_id = transactions.id

Answer 1

The reason your variable query doesn't work is that transactions.id in the WHERE clause of your variable definitions is not defined (NULL), hence the queries won't return any results.
I think you can simplify your query by putting the variables into a derived table using conditional aggregation that you then JOIN to:
SELECT AVG(transSells - transBuys) AS avgProfit,
SUM(transSells + transBuys) AS volume
FROM transactions
JOIN (SELECT transaction_id,
SUM(CASE WHEN type='closed' THEN quant * price ELSE 0 END) AS transSells,
SUM(CASE WHEN type='opened' THEN quant * price ELSE 0 END) AS transBuys
FROM order_fills) o ON o.transaction_id = transactions.id
WHERE status = 'closed'

Answer 2

I found a slightly better solution for my case:
setting variables inside the query, and then using them in the same, single query.
SET #comission = 0.03;
SELECT count(*),
AVG(
( #transSells := (SELECT sum(order_fills.quant * order_fills.price)
FROM order_fills
WHERE `type`='closed'
AND order_fills.transaction_id = transactions.id))
-
( #transBuys := (SELECT sum(order_fills.quant * order_fills.price)
FROM order_fills
WHERE `type`='opened'
AND order_fills.transaction_id = transactions.id))) AS avgProfit,
SUM( #transSells ) AS volume,
SUM( #transSells * #comission )
FROM transactions
WHERE transactions.status='closed';