SELECT  * , min(delta) FROM (
SELECT
     b.a_id,
     b.uY bduy, b.uM bdum,
     a.uY aduy, a.uM adum,
 (b.uY*12 + b.uM  - a.uY*12 - a.uM) delta,
     b.fin_km bdfk, a.fin_km adfk,
     b.fin_km- a.fin_km probeg
  FROM rashod as a
   JOIN rashod as b ON  (b.a_id = a.a_id)
 ) as d
 WHERE bdum = 8 AND delta >0
 GROUP BY a_id

DROP TABLE IF EXISTS `rashod`;
CREATE TABLE `rashod` (
  `id` int(3) NOT NULL AUTO_INCREMENT,
  `uY` int(4) NOT NULL DEFAULT '2017',
  `uM` int(2) NOT NULL,
  `a_id` int(2) NOT NULL,
  `beg_km` int(8) NOT NULL,
  `fin_km` int(8) NOT NULL,
  `beg_fuel` float NOT NULL,
  `buy_fuel_cash` float NOT NULL,
  `buy_fuel_card` float NOT NULL,
  PRIMARY KEY (`id`),
  KEY `a_id` (`a_id`)
) ENGINE=InnoDB AUTO_INCREMENT=14 DEFAULT CHARSET=utf8;

CREATE TABLE data (
car INT,
km INT,
y INT,
m INT
);

-- Месяц для поиска
SET @y = 2016;
SET @m = 8;

-- Тест-данные
INSERT INTO data (car,km,y,m)
-- есть за текущий и предыдущий месяцы, результат = 1
SELECT 1,1,2016,6 UNION ALL
SELECT 1,4,2016,7 UNION ALL
SELECT 1,5,2016,8 UNION ALL
-- есть за текущий и на 2 месяца раньше, результат = 11
SELECT 2,11,2016,1 UNION ALL
SELECT 2,16,2016,6 UNION ALL
SELECT 2,22,2016,8 UNION ALL
-- есть за текущий, нет раньше, результат = 33
SELECT 3,33,2016,8 UNION ALL
-- нет за текущий, есть раньше, результат = 0
SELECT 4,44,2016,1 ;
SELECT 4,47,2016,2 ;
SELECT 4,51,2016,3 ;
SELECT 4,56,2016,4 ;

CREATE VIEW cars AS -- список машин
SELECT DISTINCT car FROM data;

SELECT * FROM cars; -- проверка

CREATE VIEW current_km AS -- пробег на текущий месяц
SELECT cars.car, COALESCE(data.km,0) km
FROM cars
LEFT JOIN data 
ON cars.car = data.car
AND data.y = /* @y */ 2016 AND data.m = /* @m */ 8;

SELECT * FROM current_km; -- проверка

CREATE VIEW prev_ym AS -- предыдущие год и месяц с показаниями
SELECT car, MAX(data.y * 12 + data.m) ym
FROM data
WHERE (data.y * 12 + data.m) < (/* @y */ 2016 * 12 + /* @m */ 8)
GROUP BY car;

SELECT * FROM prev_ym; -- проверка

CREATE VIEW prev_km AS -- предыдущий пробег
SELECT DISTINCT cars.car, COALESCE(data.km,0) km
FROM cars
LEFT JOIN (data 
           JOIN prev_ym ON (data.y * 12 + data.m) = prev_ym.ym
          ) ON cars.car = data.car;

SELECT * FROM prev_km; -- проверка

-- Получаем итоги
SELECT cars.car, GREATEST(current_km.km - prev_km.km, 0) km
FROM cars 
JOIN current_km ON cars.car = current_km.car
JOIN prev_km ON cars.car = prev_km.car;

-- Сборка в один запрос

SELECT cars.car, GREATEST(current_km.km - prev_km.km, 0) km
FROM (SELECT DISTINCT car FROM data) cars 
JOIN (
		SELECT cars.car, COALESCE(data.km,0) km
		FROM (SELECT DISTINCT car FROM data) cars
		LEFT JOIN data 
		ON cars.car = data.car
		AND data.y = @y AND data.m = @m
	 ) current_km ON cars.car = current_km.car
JOIN (
		SELECT DISTINCT cars.car, COALESCE(data.km,0) km
		FROM (SELECT DISTINCT car FROM data) cars
		LEFT JOIN (data 
				   JOIN (
							SELECT car, MAX(data.y * 12 + data.m) ym
							FROM data
							WHERE (data.y * 12 + data.m) < (@y * 12 + @m)
							GROUP BY car				   
				        ) prev_ym ON (data.y * 12 + data.m) = prev_ym.ym
				  ) ON cars.car = data.car
     ) prev_km ON cars.car = prev_km.car;


-- Очистка
DROP VIEW prev_km;
DROP VIEW prev_ym;
DROP VIEW current_km;
DROP VIEW cars;
DROP TABLE data;