How to sum non-consecutive values when using group by in MySQL - mysql

Question

I have a data set representing alarms' state at a given timestamp (every 15 minutes). When the value is 1 the alarm is ON, 0 when OFF. I am trying to count the number of times the alarm has been triggered per hour (non-consecutive 1).
I took a look at Count max number of consecutive occurrences of a value in SQL Server but couldn't manage to adapt the answer.
Basically the data set for one alarm looks like this:
| id | value | registered_at |
| -- | ---------|---------------------|
| 1 | 1 | 2012-07-15 06:00 |
| 2 | 0 | 2012-07-15 06:15 |
| 3 | 1 | 2012-07-15 06:30 |
| 4 | 0 | 2012-07-15 06:45 |
| 5 | 1 | 2012-07-15 07:00 |
| 6 | 1 | 2012-07-15 07:15 |
| 7 | 1 | 2012-07-15 07:30 |
| 8 | 0 | 2012-07-15 07:45 |
| 8 | 0 | 2012-07-15 08:00 |
The results I am looking for is the following
| registered_at | alarm_triggered |
|--------------------|-----------------|
| 2012-07-15 06 | 2 |
| 2012-07-15 07 | 1 |
| 2012-07-15 08 | 0 |
To create groups I use EXTRACT(DAY_HOUR from registered_at).
Can you help me create the query?
(First time poster on SO, any feedback about the form of this post would be greatly appreciated as well)

Answer 1

Use LAG() window function to check the value of value of the previous row and if it is different and the current row is 1 then sum:
SELECT registered_at,
SUM(value * flag) alarm_triggered
FROM (
SELECT value,
DATE_FORMAT(registered_at, '%Y-%m-%d %H') registered_at,
value <> LAG(value, 1, 0) OVER (PARTITION BY DATE_FORMAT(registered_at, '%m-%d-%Y %H') ORDER BY registered_at) flag
FROM tablename
) t
GROUP BY registered_at
See the demo.
Results:
registered_at
alarm_triggered
2012-07-15 06
2
2012-07-15 07
1
2012-07-15 08
0

Answer 2

I assume the registered_at field is datetime so you need to use datetime function.
here is a query for this:
SELECT DATE_FORMAT(registered_at, "%Y-%m-%d %H:00:00") AS registered_at, SUM(VALUE) AS alarm_triggered
FROM ALARMS
GROUP BY DATE_FORMAT(registered_at, "%Y-%m-%d %H:00:00")
and sqlfiddle to see example:
example

Answer 3

If you need only notified days
select count(value), date_format(registered_at, '%m-%d-%Y %H') as c_at
from notifications
where value = 1
group by date_format(registered_at, '%m-%d-%Y %H');
Or all days
select sum(value), date_format(registered_at, '%m-%d-%Y %H') as c_at
from notifications
group by date_format(registered_at, '%m-%d-%Y %H');
Try this!

Answer 4

You can select it like this:
SELECT CONCAT(YEAR(registered_at), '-', MONTH(registered_at), '-', DAYOFMONTH(registered_at), ' ' HOUR(registered_at)), count(*)
FROM alarms
WHERE value = 1
GROUP BY YEAR(registered_at), MONT(registered_at), DAYOFMONTH(registered_at), HOUR(registered_at);
Explanation
First, we find the records whose value is 1, then group them by year, month, day of month and hour and finally we find out their count.