Any way to reference MySQL "AS" values within the query? - mysql

Question

in the following query, I sum up all the Sale Items of all the Sales in the given range. Included in this summary is "sumOfCost" and "sumOfPrice", I'd like "sumOfProfit" too. The problem is, as they're only "AS" variables, I'm not sure how to reference them, and my attempt of "... Blah AS blah, sumOfPrice - sumOfCost = sumOfProfit ..." sadly didn't work!
Here is my query:
SELECT Sales.SaleID,
Sales.StaffID,
Sales.CustomerID,
Sales.Timestamp,
Sales.Refunded,
Sales.PaymentType,
Staff.Forename AS staffForename,
Staff.Surname AS staffSurname,
(
SELECT GROUP_CONCAT(Quantity, ' x ', Name)
FROM SaleItems
WHERE SaleItems.SaleID = Sales.SaleID
) AS itemList,
(
SELECT sum(Cost*Quantity)
FROM SaleItems
WHERE SaleItems.SaleID = Sales.SaleID
) AS sumOfCost,
(
SELECT sum(Price*Quantity)
FROM SaleItems
WHERE SaleItems.SaleID = Sales.SaleID
) AS sumOfPrice
FROM Sales
INNER JOIN Staff
ON Sales.StaffID = Staff.StaffID
WHERE Sales.Deleted = '0'
ORDER BY Timestamp DESC
LIMIT 0, 15
Apologies for the length of the query, I've struggled before to optimise it better but right now I'm only looking to solve this problem.
Thanks in advance :)

Answer 1

One alternative to solve this problem is by wrapping the whole query with a subquery,
SELECT *,
sumOfCost - sumOfPrice AS sumOfProfit
FROM
(
SELECT Sales.SaleID,
Sales.StaffID,
Sales.CustomerID,
Sales.Timestamp,
Sales.Refunded,
Sales.PaymentType,
Staff.Forename AS staffForename,
Staff.Surname AS staffSurname,
(
SELECT GROUP_CONCAT(Quantity, ' x ', Name)
FROM SaleItems
WHERE SaleItems.SaleID = Sales.SaleID
) AS itemList,
(
SELECT sum(Cost*Quantity)
FROM SaleItems
WHERE SaleItems.SaleID = Sales.SaleID
) AS sumOfCost,
(
SELECT sum(Price*Quantity)
FROM SaleItems
WHERE SaleItems.SaleID = Sales.SaleID
) AS sumOfPrice
FROM Sales
INNER JOIN Staff
ON Sales.StaffID = Staff.StaffID
WHERE Sales.Deleted = '0'
ORDER BY Timestamp DESC
LIMIT 0, 15
) s
By the way, they are called ALIAS.
The reason why ALIAS can't be used for calculation on the same level where they were defined is because the server executes the FROM clause before the SELECT clause. The ALIAS are on the SELECT clause, here's the full SQL Order of Operation:
FROM clause
WHERE clause
GROUP BY clause
HAVING clause
SELECT clause
ORDER BY clause
For better performance issue, I'll rather using JOIN than subqueries.
UPDATE 1
SELECT Sales.SaleID,
Sales.StaffID,
Sales.CustomerID,
Sales.Timestamp,
Sales.Refunded,
Sales.PaymentType,
Staff.Forename AS staffForename,
Staff.Surname AS staffSurname,
COALESCE(a.itemList, '') itemList,
COALESCE(b.sumOfCost, 0) sumOfCost,
COALESCE(c.sumOfPrice, 0) sumOfPrice,
COALESCE(b.sumOfCost, 0) - COALESCE(c.sumOfPrice, 0) AS sumOfProfit
FROM Sales
INNER JOIN Staff
ON Sales.StaffID = Staff.StaffID
LEFT JOIN
(
SELECT SaleID, GROUP_CONCAT(Quantity, ' x ', Name) itemList
FROM SaleItems
GROUP BY SaleID
) a ON a.SaleID = Sales.SaleID
LEFT JOIN
(
SELECT SaleID, sum(Cost*Quantity) sumOfCost
FROM SaleItems
GROUP BY SaleID
) b ON b.SaleID = Sales.SaleID
LEFT JOIN
(
SELECT SaleID, sum(Price*Quantity) sumOfPrice
FROM SaleItems
GROUP BY SaleID
) c ON c.SaleID = Sales.SaleID
WHERE Sales.Deleted = '0'
ORDER BY Timestamp DESC
LIMIT 0, 15
UPDATE 2
SELECT Sales.SaleID,
Sales.StaffID,
Sales.CustomerID,
Sales.TIMESTAMP,
Sales.Refunded,
Sales.PaymentType,
Staff.Forename AS staffForename,
Staff.Surname AS staffSurname,
COALESCE(a.itemList, '') itemList,
COALESCE(a.sumOfCost, 0) sumOfCost,
COALESCE(a.sumOfPrice, 0) sumOfPrice,
COALESCE(a.sumOfCost, 0) - COALESCE(a.sumOfPrice, 0) AS sumOfProfit
FROM Sales
INNER JOIN Staff
ON Sales.StaffID = Staff.StaffID
LEFT JOIN
(
SELECT SaleID,
GROUP_CONCAT(Quantity, ' x ', Name) itemList,
SUM(Cost*Quantity) sumOfCost,
SUM(Price*Quantity) sumOfPrice
FROM SaleItems
GROUP BY SaleID
) a ON a.SaleID = Sales.SaleID
WHERE Sales.Deleted = '0'
ORDER BY TIMESTAMP DESC
LIMIT 0, 15