mysql从同一列中的不同行中减去

Question

I have a table for mileage records. Like this sample one. I need to calculate mileage usage for particular month.

Date            Meter
---------------------
2014-01-01      12500
2014-01-12      12650
2014-02-01      13000
2014-02-15      13350
2014-03-01      15000

I need an answer for example: for January the result is 500 (from the subtraction 13000-12500 ) or for February the reuslt is 2000 ( 15000-13000 ).

I tried the following but I'm getting the wrong answer. How do I get about this? Thank you.

select (select Meter from tbl where Date = '2014-12-01') - 
       (select Meter from tbl where Date = '2014-11-01');

Answer 1

Try this:

SELECT EXTRACT(MONTH FROM t1.`Date`) AS month,
       t2.Meter - t1.Meter
FROM mytable AS t1
JOIN mytable AS t2 ON t2.`Date` =  DATE_ADD(t1.`Date`, INTERVAL 1 MONTH)
WHERE EXTRACT(DAY FROM t1.`Date`) = 1

This should work as long as you have one record for each 1st day of month date.