SUM in multi-currency - sql

Question

I am trying to do SUM() in a multi-currency setup. The following will demonstrate the problem that I am facing:-
Customer
-------------------------
Id | Name
1 | Mr. A
2 | Mr. B
3 | Mr. C
4 | Mr. D
-------------------------
Item
-------------------------
Id | Name | Cost | Currency
1 | Item 1 | 5 | USD
2 | Item 2 | 2 | EUR
3 | Item 3 | 10 | GBP
4 | Item 4 | 5 | GBP
5 | Item 5 | 50 | AUD
6 | Item 6 | 20 | USD
7 | Item 3 | 10 | EUR
-------------------------
Order
-------------------------
User_Id | Product_Id
1 | 1
2 | 1
1 | 2
3 | 3
1 | 5
1 | 7
1 | 5
2 | 6
3 | 4
4 | 2
-------------------------
Now, I want the output of a SELECT query that lists the Customer Name and the total amount worth of products purchased as:-
Customer Name | Amount
Mr. A | Multiple-currencies
Mr. B | 25 USD
Mr. C | 15 GBP
Mr. D | 2 EUR
So basically, I am looking for a way to add the cost of multiple products under the same customer, if all of them have the same currency, else simply show 'multiple-currencies'. Running the following query will not help:-
SELECT Customer.Name, SUM(Item.Amount) FROM Customer
INNER JOIN Order ON Order.User_Id = Customer.Id
INNER JOIN Item ON Item.Id = Order.Product_Id
GROUP BY Customer.Name
What should my query be? I am using Sqlite

Answer 1

I would suggest two output columns, one for the currency and one for the amount:
SELECT c.Name,
(case when max(currency) = min(currency) then sum(amount)
end) as amount,
(case when max(currency) = min(currency) then max(currency)
else 'Multiple Currencies'
end) as currency
FROM Customer c INNER JOIN
Order o
ON o.User_Id = c.Id INNER JOIN
Item
ON i.Id = o.Product_Id
GROUP BY c.Name
If you want, you can concatenate these into a single string column. I just prefer to have the information in two different columns for something like this.
The above is standard SQL.

Answer 2

I think your query should looks like this
SELECT
Data.Name AS [Customer Name],
CASE WHEN Data.Count > 1 THEN "Multiple-currencies" ELSE CAST(Data.Amount AS NVARCHAR) END AS Amount
FROM
(SELECT
Customer.Name,
COUNT(Item.Currency) AS Count,
SUM(Item.Amount) AS Amount
FROM
Customer
INNER JOIN Order ON Order.User_Id = Customer.Id
INNER JOIN Item ON Item.Id = Order.Product_Id
GROUP BY
Customer.Name) AS Data
A subquery to get the count of currencies and then ask for them in the main query to show the total or the text "Multiple-currencies".
Sorry if there is any mistake or mistype but I don't have a database server to test it
Hope this helps.

Answer 3

IMO I would start by standardizing variable names. Why call ID in customer table USER_ID in order table? Just a pet peeve. Anyway, you should learn how to build queries.
start with joining the customer table to the order table on then join the result to the item table. The first join is on CUSTOMER_ID and the second join is on PRODUCT_ID. Once you have that working use SUM and GROUP BY

Answer 4

Ok, I managed to solve the problem this way:-
SELECT innerQuery.Name AS Name, (CASE WHEN innerQuery.Currencies=1 THEN (innerQuery.Amount || innerQuery.Currency) ELSE 'Mutliple-Currencies' END) AS Amount, FROM
(SELECT Customer.Name, SUM(Item.Amount), COUNT(DISTINCT Item.Currency) AS Currencies, Item.Currency AS Currency FROM Customer
INNER JOIN Order ON Order.User_Id = Customer.Id
INNER JOIN Item ON Item.Id = Order.Product_Id
GROUP BY Customer.Name)innerQuery