How to combine two sql expressions into one? - mysql

Question

I have the following issue:
It is necessary to write a query that will output 'item_id', 'price_in_byr'.
'price_in_byr' is calculated as the 'price' of the items table multiplied by the currency rate at the maximum date of the rate from the table rates.
See Schema
I apologize for my English, I'll try to explain by example:
Goods with item_id = 5 costs 20 euros, in the rates table the maximum date for the euro is January 12, at that date the exchange rate was 25. Total our 'price_in_byr' is 25 * 20 = 500
My solution with temp table:
CREATE TABLE tempRate SELECT currency, MAX(rate) AS maxRate FROM rates GROUP
BY currency;
SELECT items.item_id,(ifnull(tempRate.maxRate,1) * items.price) AS price_in_byr
FROM items
LEFT JOIN tempRate ON items.currency = tempRate.currency;
Tell me please, how can I do it in one query?

Answer 1

You can just use a subquery:
SELECT
items.item_id,(ifnull(tempRate.maxRate,1) * items.price) AS price_in_byr
FROM
items
LEFT JOIN
(
SELECT
currency, MAX(rate) AS maxRate
FROM
rates
GROUP BY
currency
) AS tempRate
ON items.currency = tempRate.currency;
In practice, you substitute "tempRate" by (definition of tempRate) AS tempRate.
You can see an example at dbfiddle here
If you actually want the * most_recent_rate*, you'd do something completely different; insert a subquery to compute it. This subquery looks for all rates of the given currency, sorts them by their exchange_ts (meaning timestamp) in descending order, and just picks to top 1:
SELECT
items.item_id,(ifnull(
(
SELECT
rate AS most_recent_rate
FROM
rates
WHERE
rates.currency = items.currency
ORDER BY
exchange_ts DESC
LIMIT 1
)
, 1) * items.price) AS price_in_byr
FROM
items ;
dbfiddle here

Answer 2

You can make your tempRate query into a subquery:
SELECT
items.item_id,
(ifnull(tempRate.maxRate,1) * items.price) AS price_in_byr
FROM
items
LEFT JOIN (
SELECT currency, MAX(rate) AS maxRate
FROM rates
GROUP BY currency
) as tempRate ON items.currency = tempRate.currency;

Answer 3

I am not sure if mysql supports this level of subquery but see if it works:
select I.item_id, I.price * CR.rt
from
items as I,
(
select r.currency cy, r.rate rt
from
rates as r, (select currency, max(date) max_date from rates group by currency) as R_MAXDATES
where
r.currency = R_MAXDATES.currency
and r.date = R_MAXDATES.max_date;
) As CR
where
I.currency = CR.cy