SQL 在每月聚合中查找最新的子状态条目

Question

我在 BigQuery 数据库中有两个表：“project”和“project_status”...

project
Id  Name
1   XXXX
2   YYYY
3   ZZZZ

project_status
project_id   status_date  status
1              2 Jan 2020    Good
2              4 Feb 2020    Bad
3              5 Feb 2020    On track
3              6 March 2020  Bad

我想制作一份项目状态的月度报告。项目的状态不会每月更新。在这种情况下，截至月底的最新状态条目将被视为最新状态

Month       Project  Status
Jan 2020      XXXX     Good
Feb 2020      XXXX     Good
Mar 2020      XXXX     Good
Jan 2020      YYYY     null
Feb 2020      YYYY     Bad
Mar 2020      YYYY     Bad
Jan 2020      ZZZZ     null
Feb 2020      ZZZZ     On track
Mar 2020      ZZZZ     Bad

例如，对于项目“XXXX”，2020 年 2 月或 2020 年 3 月没有状态更新，因此状态将取自 2020 年 1 月 2 日的最新状态条目。

关于如何构建 SQL 查询以提供此报告的任何建议？

Answer 1

考虑使用GENERATE_DATE_ARRAY AND ARRAY_AGG：

WITH Months AS (
  SELECT DATE_SUB(month, INTERVAL 1 DAY) AS last_day
  FROM UNNEST(GENERATE_DATE_ARRAY('2020-02-01', '2020-04-02', INTERVAL 1 MONTH)) AS month
),
Projects as (
  SELECT 1 AS Id, 'XXXX' AS Name UNION ALL
  SELECT 2 AS Id, 'YYYY' AS Name UNION ALL
  SELECT 3 AS Id, 'ZZZZ' AS Name
),
Project_Status as (
  SELECT 1 AS project_id, PARSE_DATE('%e %b %Y', '2 Jan 2020') AS status_date, 'Good' AS status UNION ALL
  SELECT 2, PARSE_DATE('%e %b %Y', '4 Feb 2020'), 'Bad' UNION ALL
  SELECT 3, PARSE_DATE('%e %b %Y', '5 Feb 2020'), 'On track' UNION ALL
  SELECT 3, PARSE_DATE('%e %b %Y', '6 March 2020'), 'Bad'
)
SELECT Projects.Name as Name, DATE_TRUNC(Months.last_day, MONTH) AS Month,
  ARRAY_AGG(Project_Status.status ORDER BY status_date DESC)[OFFSET(0)]  AS Status
FROM Projects 
CROSS JOIN Months 
LEFT JOIN Project_Status ON 
  Project_Status.project_id = Projects.id
  AND Project_Status.status_date <= Months.last_day
GROUP BY Name, Month

输出是：