postgresql - 在 postgres/COALESCE 中正确使用 COALESCE 无法正常工作

Question

我正在尝试返回有关所执行搜索次数的每小时报告。我的结果不包括搜索为零的时间，我认为我的语法正确，可以使用 COALESCE。谁能看到我做错了什么？谢谢

SELECT CAST(startdatetime as Date),extract(hour from startdatetime) as hr, COALESCE(count(distinct id),0) as average_per_hour
FROM search WHERE CAST(startdatetime As Date) = '2014/07/05' 
GROUP BY CAST(startdatetime as Date),extract(hour from startdatetime) 
ORDER BY CAST(startdatetime as Date),extract(hour from startdatetime)

Answer 1

一些改进，但与@a_horse_with_no_name 的回答基本相同:

SELECT DATE '2014-07-05', hr, COUNT(DISTINCT id) AS average_per_hour
FROM generate_series(0, 23) hr
LEFT JOIN search ON EXTRACT(HOUR FROM startdatetime) = hr AND CAST(startdatetime AS DATE) = '2014-07-05' 
GROUP BY hr
ORDER BY hr

在 ORDER BY 和 GROUP BY 中使用 CAST(startdatetime AS DATE) 是无关紧要的，因为您只搜索一天。如果通常情况并非如此，您还需要调整 generate_series()。

编辑:

这适用于多天:

SELECT CAST(hr AS DATE), EXTRACT(HOUR FROM hr), COUNT(DISTINCT id) AS average_per_hour
FROM generate_series('2014-07-05 00:00:00', '2014-07-06 23:00:00', INTERVAL '1' HOUR) hr
LEFT JOIN search ON date_trunc('hour', startdatetime) = hr
GROUP BY hr
ORDER BY hr