MySQL counts the number of records per hour by time

MySQL time-per-hour record number scheme 1:?

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SELECT@rownum := @rownum + 1 AS ID,        CONCAT((CASE WHEN t.hour = 24 THEN 0 ELSE t.hour END),‘:00:00‘) AS TIME, COUNT(*) AS COUNTFROM (SELECT id, HOUR(DATE_FORMAT(createtime,‘%H:%i:%s‘))+1 AS HOUR FROM baby WHERE DATE_FORMAT(createtime,‘%Y-%m-%d‘)=‘2016-10-16‘) t,     (SELECT @rownum := 0) rGROUP BY TIME ORDER BY ID//这个计数, 是LESS THAN TIME的计数方法, 即11:xx:xx的记录是记到12:00:00下面的, 而不是11:00:00;

The above modification?

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SELECT@rownum := @rownum + 1 AS ID,        CONCAT((CASE WHEN t.hour = 24 THEN 0 ELSE t.hour END),‘‘) AS TIME, COUNT(*) AS COUNTFROM (SELECT id, HOUR(DATE_FORMAT(createtime,‘%H:%i:%s‘)) AS HOUR FROM baby WHERE DATE_FORMAT(createtime,‘%Y-%m-%d‘)=‘2016-10-16‘) t,     (SELECT @rownum := 0) rGROUP BY TIME ORDER BY ID

Scenario 2?

1	SELECTLEFT( createtime, 13 ) , COUNT( * ) FROM baby WHERE DATE_FORMAT(createtime,‘%Y-%m-%d‘)=‘2016-10-16‘ GROUP BYLEFT( createtime, 13 )

If the current hour no data currently cannot return 0! I don't know how to handle it.

MySQL counts the number of records per hour by time