Mysql query for calculating power outage between two time stamps

Question

This is a revised version of my question.

I am currently designing a simple power quality monitoring tool. I have managed to design a MySQL database and populate the table with the voltage status and the corresponding time stamp taken at particular times. Below is my table structure.

+----+---------------------+-------------+
| Id | Time_Stamp          | Red_Ph_Volt |
+----+---------------------+-------------+
|  1 | 2015-02-01 17:33:45 |      250.00 |
|  2 | 2015-02-01 18:53:41 |      250.00 |
|  3 | 2015-02-01 18:54:39 |       25.00 |
|  4 | 2015-02-01 18:54:54 |      242.00 |
|  5 | 2015-02-01 18:55:11 |      222.00 |
|  6 | 2015-02-02 21:00:29 |      250.00 |
|  7 | 2015-02-02 21:00:45 |      220.00 |
|  8 | 2015-02-02 21:00:55 |      230.00 |
|  9 | 2015-02-02 21:03:01 |      230.00 |
| 10 | 2015-02-02 21:03:36 |      250.00 |
| 11 | 2015-02-02 21:03:46 |       50.00 |
| 12 | 2015-02-06 17:54:08 |        0.00 |
| 13 | 2015-02-06 23:04:04 |      220.00 |
| 14 | 2015-02-06 23:04:34 |      220.00 |
| 15 | 2015-02-06 23:05:51 |      250.00 |
| 16 | 2015-02-08 16:04:44 |      220.00 |
| 17 | 2015-02-08 16:06:29 |      220.00 |
| 18 | 2015-02-09 09:04:12 |      220.00 |
| 19 | 2015-02-09 10:42:39 |      203.00 |
| 20 | 2015-02-09 19:34:43 |      203.00 |
| 21 | 2015-02-09 21:57:02 |      203.00 |
| 22 | 2015-02-10 09:47:08 |        0.00 |
| 23 | 2015-02-10 11:15:34 |      250.00 |
| 24 | 2015-02-10 11:48:14 |      250.00 |
| 25 | 2015-02-10 13:18:14 |      220.00 |
| 26 | 2015-02-10 18:59:52 |        0.00 |
| 27 | 2015-02-10 22:44:14 |      250.00 |
| 28 | 2015-02-10 22:47:10 |      212.00 |
| 29 | 2015-02-14 00:02:10 |      212.00 |
| 30 | 2015-02-14 00:28:57 |      242.00 |
| 31 | 2015-02-14 00:35:56 |       21.00 |
| 32 | 2015-02-16 12:11:47 |       21.00 |
+----+---------------------+-------------+
32 rows in set (0.02 sec)

Any voltage below 50V is taken as an outage. I want to write a query that queries the table for the TOTAL OUTAGE TIME between 2015-02-01 17:33:45 and 2015-02-14 00:28:57 .

Guys, kindly help me on this because I really don't know how to approach it.

Answer 1

This should work:

SELECT sum(timestampdiff(SECOND, Time_Stamp, (SELECT Time_Stamp
                                              FROM voltage AS V2
                                              WHERE V2.Id > V1.Id
                                              LIMIT 1))) AS T
FROM voltage AS V1
WHERE Red_Ph_Volt <= 50 AND Time_Stamp BETWEEN '2015-02-01 17:33:45' AND '2015-02-14 00:28:57';

I have assumed that a voltage of 50 is also considered as an outage and I have assigned the table name as voltage.

Answer 2

The first step to properly address this kind of queries is to join every row with its previous row.

   SELECT *
     FROM voltage AS V
LEFT JOIN voltage AS PRV_V ON (PRV_V.Id = V.Id-1)

Now you just have to keep those rows which its previous measure was below 50 and sum their timestamp differences.

   SELECT SUM(TIMESTAMPDIFF(SECOND, PRV_V.Time_Stamp,  V.Time_Stamp))
     FROM voltage AS V
LEFT JOIN voltage AS PRV_V ON (PRV_V.Id = V.Id-1)
    WHERE PRV_V.Red_Ph_Volt < 50
      AND V.Time_Stamp BETWEEN '2015-02-01 17:33:45' AND '2015-02-14 00:28:57'

EDIT: If your data don't have autoincremental ids or you have gaps between your ids you can still join your measurements with its previous measurement using the timestamp:

   SELECT *
     FROM voltage AS V
LEFT JOIN voltage AS PRV_V 
       ON (  PRV_V.Time_Stamp < V.Time_Stamp
             AND NOT EXISTS ( SELECT *  
                                FROM voltage 
                               WHERE Time_Stamp > PRV_V.Time_Stamp
                                 AND voltage.Time_Stamp < V.Time_Stamp
                            )
          )