如何在一个值之前的值上计算聚合函数 COUNT(DISTINCT)？

Posted 2023-03-31

【中文标题】如何在一个值之前的值上计算聚合函数 COUNT(DISTINCT)？

【英文标题】：How to calculate aggregate function COUNT(DISTINCT ) over values previous to one value?

【发布时间】：2019-07-02 04:04:32

【问题描述】：

我在 Google BigQuery 上有员工记录，其中包含：employee_identifier、manager_identifier 和 date_of_the_record

我的目标是通过 SQL 查询为每条记录计算员工在记录日期之前拥有的经理人数。

我尝试过不同的子句：OVER (PARTITION BY / ROWS UNBOUNDED PRECEDING) 等。

我试过的是：

SELECT 
  employee_identifier, 
  date_of_the_record,
  COUNT(DISTINCT manager_identifier) 
    OVER (PARTITION BY employee_identifier ORDER BY date_of_the_record ROWS UNBOUNDED PRECEDING) AS number_of_managers_until_date_of_the_record
FROM employee_database

但DISTINCT 子句禁止使用ORDER BY。

总而言之，我只想要一个员工在记录日期之前拥有的（不同的）经理的数量。

【参考方案1】：

您可以使用correlated sub-query 实现此目的，以下查询应该可以满足您的需求

CREATE TABLE #emp (employee_identifier INT,date_of_the_record DATE,manager_identifier INT)

INSERT INTO #emp VALUES
(1,getdate()-90,10),
(1,getdate()-80,20),
(1,getdate()-70,30),
(1,getdate()-60,10),
(1,getdate()-30,40),
(1,getdate()-20,80)

SELECT 
employee_identifier, 
date_of_the_record,
(SELECT COUNT(DISTINCT (manager_identifier)) FROM #emp e WHERE e.employee_identifier = emp.employee_identifier AND e.date_of_the_record <= emp.date_of_the_record) AS number_of_managers_until_date_of_the_record
FROM #emp emp
GROUP BY employee_identifier, 
date_of_the_record

结果如下，

employee_identifier date_of_the_record  number_of_managers_until_date_of_the_record
1                   2019-04-03          1
1                   2019-04-13          2
1                   2019-04-23          3
1                   2019-05-03          3
1                   2019-06-02          4
1                   2019-06-12          5

【参考方案2】：

以下是 BigQuery 标准 SQL

#standardSQL
SELECT * EXCEPT(arr),
  (SELECT COUNT(DISTINCT id) FROM UNNEST(arr) id) AS number_of_managers_until_date_of_the_record
FROM (
  SELECT *, ARRAY_AGG(manager_identifier) OVER(win) arr
  FROM `project.dataset.employee_database`
  WINDOW win AS (PARTITION BY employee_identifier ORDER BY date_of_the_record)
)