避免BigQuery中的相关子查询错误 [英] Avoid correlated subqueries error in BigQuery

问题描述

我有一个简单的查询来获取创建交易时使用的货币汇率:

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.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联接和命名子查询，但是似乎都没有用.做到这一点的最佳方法是什么?似乎是一个非常常见的场景，应该很容易在BQ的标准Sql中实现.

I've tried with several types of joins and named subqueries, but none seem to work. What is the best way to accomplish this? Seems like a very common scenario that should be quite straightforward to implement in BQ's Standard Sql.

推荐答案

以下是BigQuery标准SQL

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

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