Calculating difference on datetime row betwen rows on the same table

Question

I have table that holds records with tasks, status and time when triggered:

Table tblwork:

+-------------+------------+-----------------         


        

Answer 1

Assuming there is always a start for each pause and end, wouldn't something like this be more direct?

SELECT t.task
   , SUM(TO_SECONDS(t.stime) 
         * CASE WHEN t.status IN (1) THEN -1
                WHEN t.status IN (2, 3) THEN 1
                ELSE 0
           END
     ) AS totalTimeSecs
FROM tblwork AS task
GROUP BY t.task

I'm not quite sure offhand how big the values that come out of TO_SECONDS() are for current timestamps; but if they are an issue when being summed, if could be changed to

   , SUM((TO_SECONDS(t.stime) - some_constant_just_before_or_at_your_earliest_seconds)
         * CASE WHEN t.status IN (1) THEN -1
                WHEN t.status IN (2, 3) THEN 1
                ELSE 0
           END
     ) AS totalTimeSecs

---

You can detect "abnormal" data by adding the following to the select expression list

, CASE WHEN SUM(CASE 
                WHEN t.status IN (1) THEN -1 
                WHEN t.status IN (2, 3) THEN 1 
                ELSE 0 END
              ) = 0 
       THEN 'OK' 
       ELSE 'ABNORMAL' 
   END AS integrityCheck

Note: any "unclosed" intervals will be marked as abnormal; without much more complicated and expensive start and end checking for intervals to differentiate "open" from "invalid", it's probably the best that can be done. The sum used for additonal "integrityCheck" equaling -1 might hint at an open ended interval, but could also indicate an erroneous double-start.