Avoid correlated subqueries error in BigQuery

Question

I have a simple query to obtain the currency rate in use at the time a transaction was created:

SELECT t.orderid, t.date, 
 (SELECT rate FROM sources.currenc         


        

Answer 1

Below is for BigQuery Standard SQL

#standardSQL
SELECT 
  t.orderid AS orderid, 
  t.date AS date, 
  ARRAY_AGG(r.rate ORDER BY r.date LIMIT 1)[SAFE_OFFSET(0)] AS rate
FROM `sources.transactions` AS t
JOIN `sources.currency_rates` AS r
ON currencyid = 1 
AND r.date >= t.date
GROUP BY orderid, date