Question

I have a problem that I just cannot seem to get my head around, and hope someone can help give me some advice.

Ever since getting solar PV cells fitted on my house roof, I have been generating electricity and in accordance to some (rather generous) incentives to do this kind of thing, have been making money for every kWh of electricity I generate. Seeing this as being a bit of a database project, I set about writing some PHP/MySQL to track daily generation, and now have nearly a year’s worth of daily kWh readings, which are nicely presented to me in graphical form, both in a month-by-month view, and as a yearly (grouped into months) graph.

I’m now wanting to expand the system to show revenue in monetary terms, rather than kWh of electricity. Currently, the figure is £0.454 per kWh, though this figure changes every year on the April 1st (it was £0.433 previously).

This is my current MySQL structure:

Table feedin:
  year (year4)   rate (float)
  2010           0.433
  2011           0.433
  2012           0.454

Table generation:
  day (DATE)     reading (float)
  2011-12-01     7.682
  2011-12-02     5.747
  2011-12-03     4.982
  ...            ...
  2012-08-13     8.022
  2012-08-14     19.449
  2012-08-15     5.484

My first attempt at this was all rather cumbersome with a very mixed mess of PHP and MySQL queries, with the bulk of the logic being done in PHP (my MySQL skills are “limited”, at best). However, as time is going on, I see that it would be ideal if the whole thing were done in MySQL.

I’ve no real idea how to tackle this. My initial thoughts are that we need to select yearly chunks of data (well, date-ranges from April 1st in one year, to March 31st the next), and multiply it by the appropriate year rate. And that “appropriate year rate is the rate applicable at the start of that date range, ie, as of April 1st).

Ideally, I’d like the query to be able to cope with multiple yearly boundaries, so, for example, several years down the road, I’d like to be able to query the absolute total revenue produced to date. Ultimately, I would just like to pass the query the start and end dates, and it returns the correct figure.

Answer 1

Link the year of the generation date to the year of the feedin tariff

SELECT *, generation.reading*feedin.rate AS profit
 FROM generation, feedin
 WHERE YEAR(generation.day)=feedin.year

BUT as this must relate to year start of APRIL 1st

SELECT *, generation.reading*feedin.rate AS profit
 FROM generation, feedin
 WHERE YEAR(DATE_SUB(generation.day, INTERVAL 3 MONTH))=feedin.year

This will move the recorded dates back 3 months too, making them Jan-Dec instead of Apr-Mar wich will then match the feedin year