google-bigquery - 避免 BigQuery 中的相关子查询错误

Question

我有一个简单的查询来获取创建交易时使用的货币汇率:

SELECT t.orderid, t.date, 
 (SELECT rate FROM sources.currency_rates r WHERE currencyid=1 AND 
r.date>=t.date  ORDER BY date LIMIT 1) rate
FROM sources.transactions t

这会触发一个错误:

Error: Correlated subqueries that reference other tables are not 
supported unless they can be de-correlated, such as by transforming 
them into an efficient JOIN.' 

我尝试了几种类型的连接和命名子查询，但似乎都不起作用。实现这一目标的最佳方法是什么？似乎是一个非常常见的场景，在 BQ 的标准 Sql 中实现起来应该非常简单。

Answer 1

下面是 BigQuery 标准 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