Select query with calculated field that uses subquery with multiple conditions - mysql

Question

Three Tables:
COST_SAVINGS: COST_SAVINGS_ID, ONE_TIME_CREDIT, CREATION_DATE, INVOICE_ID (FK to Invoice table)
INVOICE: INVOICE_ID, INVOICE_CURRENCY_CODE
EXCHANGE_RATE: CURRENCY_RATE, CURRENCY_DATE
I'm reporting on Cost Savings (the first table). The challenge is that each cost savings amount can be in a different currency so I need a field that shows the converted amount based on the currency from the invoice table and a matching month / year between the Exchange.Ex_Date and Cost_Savings.Create_Date.
I'm getting an error that states:
single-row subquery returns more than one row
This is what I have so far:
SELECT
COST_SAVINGS.COST_SAVINGS_ID,
COST_SAVINGS.CLAIM_TYPE,
COST_SAVINGS.COMMENTS,
COST_SAVINGS.COST_SAVINGS_STATUS,
COST_SAVINGS.CREATION_DATE,
COST_SAVINGS.DESCRIPTION,
COST_SAVINGS.ONE_TIME_CREDIT AS CREDIT_IN_NATIVE_CURRENCY,
FINANCE_INVOICE.CURRENCY_CODE,
COST_SAVINGS.ONE_TIME_CREDIT *
(SELECT EXCHANGE_RATE.CURRENCY_RATE
FROM EXCHANGE_RATE
WHERE EXTRACT (MONTH FROM COST_SAVINGS.CREATION_DATE) = EXTRACT (MONTH FROM EXCHANGE_RATE.CURRENCY_DATE)
AND EXTRACT (YEAR FROM COST_SAVINGS.CREATION_DATE) = EXTRACT (YEAR FROM EXCHANGE_RATE.CURRENCY_DATE)
AND FINANCE_INVOICE.CURRENCY_CODE = EXCHANGE_RATE.CURRENCY_CODE) AS CREDIT_IN_USD
FROM COST_SAVINGS
LEFT JOIN FINANCE_INVOICE ON COST_SAVINGS.INVOICE_ID = FINANCE_INVOICE.INVOICE_ID
I feel like the issue may be with the third WHERE clause in my subquery (Trying to match the Currency codes). I'm not sure how resolve it though. Any thoughts?

Answer 1

Try putting LIMIT 1 in your subquery:
...RENCY_CODE = EXCHANGE_RATE.CURRENCY_CODE LIMIT 1) AS CREDIT_IN_USD
I believe your subquery is returning multiple rows. That doesn't work when you use a subquery in place of a column name in your SELECT clause.
If you don't want to use a subquery in place of a column name, try something like this. You'll join to a subquery generated virtual table.
Here's the virtual table for exchange rates. It uses GROUP BY to make one row (or no rows per month/year/currency code. If more than one row is in the raw table for any month/year/code, they get averaged with AVG(). You could also use MAX() or MIN().
SELECT AVG(CURRENCY_RATE) CURRENCY_RATE,
CURRENCY_CODE,
EXTRACT(MONTH FROM CURRENCY_DATE) MONTH,
EXTRACT(YEAR FROM CURRENCY_DATE) YEAR
FROM EXCHANGE_RATE
GROUP BY CURRENCY_CODE,
EXTRACT(MONTH FROM CURRENCY_DATE),
EXTRACT(YEAR FROM CURRENCY_DATE)
Try this and convince yourself it works.
Then build it into your overall query.
SELECT
COST_SAVINGS.COST_SAVINGS_ID,
COST_SAVINGS.whatever, ...
FINANCE_INVOICE.CURRENCY_CODE,
(COST_SAVINGS.ONE_TIME_CREDIT * RATE.CURRENCY_RATE) AS CREDIT_IN_USD
FROM COST_SAVINGS
LEFT JOIN FINANCE_INVOICE ON COST_SAVINGS.INVOICE_ID = FINANCE_INVOICE.INVOICE_ID
LEFT JOIN (SELECT AVG(CURRENCY_RATE) CURRENCY_RATE,
CURRENCY_CODE,
EXTRACT(MONTH FROM CURRENCY_DATE) MONTH,
EXTRACT(YEAR FROM CURRENCY_DATE) YEAR
FROM EXCHANGE_RATE
GROUP BY CURRENCY_CODE,
EXTRACT(MONTH FROM CURRENCY_DATE),
EXTRACT(YEAR FROM CURRENCY_DATE)
) RATE ON RATE.CURRENCY_CODE = FINANCE_INVOICE.CURRENCY_CODE
AND RATE.YEAR = EXTRACT(YEAR FROM COST_SAVINGS.CREATION_DATE)
AND RATE.MONTH = EXTRACT(MONTH FROM COST_SAVINGS.CREATION_DATE)