缺少日期的 PostgreSQL 聚合

Question

我正在尝试这个查询

SELECT date_trunc('day', commit_at) AS day, count(*)
  FROM commits
  GROUP BY date_trunc('day', commit_at)
  ORDER BY date_trunc('day', commit_at) ASC;

它返回

      day         | count 
---------------------+-------
2015-05-18 00:00:00 |     5
2015-05-19 00:00:00 |     2
2015-05-21 00:00:00 |     2
(3 lignes)

问题是：我如何才能强制将空天数包含在结果中？

      day         | count 
---------------------+-------
2015-05-18 00:00:00 |     5
2015-05-19 00:00:00 |     2
2015-05-20 00:00:00 |     0
2015-05-21 00:00:00 |     2
(3 lignes)

Answer 1

一种方法是使用 nugenerate_series生成最小日期和最大日期之间的所有天数，然后将其连接到聚合查询：

SELECT    DATE_TRUNC ('day', 
                      GENERATE_SERIES (MIN(commit_at), MAX(commit_at), '1 day') 
             AS day, 
          COALESCE (cnt, 0)
FROM      commits
LEFT JOIN (SELECT   DATE_TRUNC('day', commit_at) AS cday
           FROM     commits
           GROUP BY DATE_TRUNC('day', commit_at)) agg ON day = cday
ORDER BY  1 ASC