Question

all! I have the following problem: I have 2 simple tables. The first one, say, “currency”, stores the information about the currencies (ISO 4217 code (PK), name, etc.). The second one, “rates”, has the following fields: id, currency_code (references currency.code), date_added (datetime), rate.

What I want to fo is to select the latest exchange rates for several currency codes. First, I wanted to do it like this:

SELECT r.rate
FROM rates r
WHERE r.id IN (SELECT id
               FROM rates r1
               WHERE r1.currency_code IN('USD', 'EUR')
               GROUP BY r1.currency_code
               ORDER BY r1.date_added DESC)
ORDER BY r.currency_code ASC

but I soon remembered that the ORDER clause will be taken into account after the results are grouped. In my case, the subquery ended up returning the ids of the earliest rates…

I suppose I could get the rates for each currency code separately, but I still hope that I’ll be able to use a single clean query.

Answer 1

Assuming that the latest exchange rate is the one with the highest id you can use:

SELECT *
FROM rates r
WHERE r.id IN (
    SELECT MAX(r1.id)
    FROM rates r1
    GROUP BY r1.currency_code
) T;

But I strongly suggest another pattern I love. I explained it in another answer this morning:

SELECT
  c.*,
  r1.*
FROM currency c
INNER JOIN rates r1 ON c.code = r1.currency_code
LEFT JOIN rates r2 ON r1.currency_code = r2.currency_code AND r2.id > r1.id
WHERE r2.id IS NULL;