如何計算一個值之前的值的聚合函數 COUNT(DISTINCT )？

Question

我在 Google BigQuery 上有員工記錄，其中包含：employee_identifier、manager_identifier 和 date_of_the_record

我的目標是通過 SQL 查詢為每條記錄計算員工在記錄日期之前擁有的經理人數。

我嘗試了不同的子句：OVER (PARTITION BY / ROWS UNBOUNDED PRECEDING) 等。

我嘗試過的是：

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ORDER BY禁止DISTINCT子句。

總而言之，我只想要員工在記錄日期之前擁有的（不同的）經理的數量。

Answer 1

您可以使用correlated sub-query來實現這一點，以下查詢應該做你想做的

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

結果如下，

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

Answer 2

下面是 BigQuery 標准 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)
)