Kelompokkan menurut harus mengembalikan 0 saat mengelompokkan berdasarkan jam. Bagaimana cara melakukannya?

Anda akan membutuhkan tabel yang berisi 24 jam, lalu JOIN untuk itu. Jika Anda tidak bisa membuat tabel, Anda bisa membuatnya di dalam kueri Anda. Lihat demo ini .

SELECT a.hr, IFNULL(b.hourCount, 0) hourCount
FROM 
  (SELECT 1 AS hr
  UNION ALL
  SELECT 2
  UNION ALL
  SELECT 3
  UNION ALL
  SELECT 4
  UNION ALL
  SELECT 5
  UNION ALL
  SELECT 6
  UNION ALL
  SELECT 7
  UNION ALL
  SELECT 8
  UNION ALL
  SELECT 9
  UNION ALL
  SELECT 10
  UNION ALL
  SELECT 11
  UNION ALL
  SELECT 12
  UNION ALL
  SELECT 13
  UNION ALL
  SELECT 14
  UNION ALL
  SELECT 15
  UNION ALL
  SELECT 16
  UNION ALL
  SELECT 17
  UNION ALL
  SELECT 18
  UNION ALL
  SELECT 19
  UNION ALL
  SELECT 20
  UNION ALL
  SELECT 21
  UNION ALL
  SELECT 22
  UNION ALL
  SELECT 23
  UNION ALL
  SELECT 0) a
LEFT JOIN
  (SELECT COUNT(id) AS hourCount, HOUR(date) AS hr
   FROM `testing`
   GROUP BY HOUR(date)) b ON b.hr = a.hr