Question

I have a table used to enter weekly Hobbs hour meter readings for vehicles. I need to be able to add up the total hours used for each vehicle over a period of time. The problem I am having is that sometimes the meters get swapped out once they fail and are replaced with new meters the are not programmable and I have to start at zero hours so I will need to loop through each meter reading and determine if a meter had been swapped and account for it in my code. I’m not big on cursors but may have to use one to create a stored procedure that will work unless someone has a better solution I should try. I’m trying to think of a way a CTE might help but nothing yet.

The table structure is:

ReadingDate Datetime
EquipmentId Int
MeterReading  decimal(18,2)

I.m planning on Creating a accumulation variable, loop through the rows in a cursor and add the reading difference into the variable.
So as I loop through and see a meter reading is less than the previous reading, I must have a meter swap out and will have to account for it. I’m thinking that if I was to look ahead at the next reading and it is lower than the current, I would just skip the summing action one time and then start adding the difference to the accumulation variable. Any better way to do this? Thanks.

Answer 1

This will work in SqlServer:

SELECT t1b.EquipmentId, 
       t1a.ReadingDate As StartDate, 
       t1b.ReadingDate AS EndDate,
       CASE 
         WHEN t1b.Reading >= t1a.Reading THEN t1b.Reading - t1a.Reading 
         ELSE t1b.Reading
      END AS Hours
FROM MeterReadings AS t1a
JOIN (select t11.EquipmentId, 
             t11.ReadingDate AS Date1, 
             (SELECT MIN(t13.ReadingDate)  
                FROM MeterReadings AS t13 
               WHERE t13.EquipmentId = t11.EquipmentId 
                AND t13.ReadingDate > t11.ReadingDate 
           Group By t13.EquipmentId) AS NextReadingDate
       from MeterReadings AS t11) AS rd ON t1a.EquipmentId = rd.EquipmentId 
                                        AND t1a.ReadingDate = rd.Date1
JOIN MeterReadings as t1b ON t1b.EquipmentId = t1a.EquipmentId 
                          AND t1b.ReadingDate = rd.NextReadingDate
order by t1a.EquipmentId, t1a.readingdate

SQL Fiddle I was playing with