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

Answer 2

I've noticed similar behavior with other correlated subqueries. They are useful, but can't always be automatically modeled to JOINs by BigQuery.

Similar case which works:

#standardSQL
SELECT name, (
  SELECT AVG(temp) 
  FROM `bigquery-public-data.noaa_gsod.gsod2017` b
  WHERE a.usaf=b.stn  
) temp
FROM `bigquery-public-data.noaa_gsod.stations` a
LIMIT 10

Doesn't work:

#standardSQL
SELECT name, (
  SELECT temp 
  FROM `bigquery-public-data.noaa_gsod.gsod2017` b
  WHERE a.usaf=b.stn  
  ORDER BY da 
  LIMIT 1
) temp
FROM `bigquery-public-data.noaa_gsod.stations` a
LIMIT 10

Fix:

#standardSQL
SELECT name, ARRAY_AGG(temp ORDER BY da LIMIT 1) temp
FROM `bigquery-public-data.noaa_gsod.stations` a
JOIN `bigquery-public-data.noaa_gsod.gsod2017` b
ON a.usaf=b.stn  
GROUP BY 1
LIMIT 10

(give me a public dataset, and I'll write a query that works with your data)