Mysql difference between rows - mysql

Question

i want to get price difference of car from 2 row through given following data.
i want to substract price column ex: (200-100),(300-200) and so on as data
My Table:
My desired output:
what i have tried
select t1.row_num1,t1.car_name
from
(
select (#row_num := #row_num +1) as row_num1 ,(select #row_num =0) r1, car_name,price
from car
)t1
I know that i don't have id column.hence i am generating row_number.
now i am getting problem to self join this table and get difference.
your help is appreciable.

Answer 1

Try This
set #next_row_price := null;
SELECT car_name , price, diff FROM(
SELECT car_name,price,(#next_row_price - price) * -1 AS diff,
IF(#next_row_price IS NULL, #next_row_price := price, 0) ,
IF(#next_row_price IS NOT NULL, #next_row_price := price, 0)
FROM car
) AS TEMP;
SQLFiddle

Answer 2

Although your output seems confusing nevertheless I am giving the following answer:
SOLUTION #1
SELECT
carsTable1.car_name,
carsTable1.price,
CASE WHEN ABS(carsTable1.price - (SELECT price FROM cars WHERE car_name='car 2')) = 0 THEN NULL ELSE
ABS(carsTable1.price - (SELECT price FROM cars WHERE car_name='car 2')) END diff
FROM
(SELECT
#rn := #rn + 1 row_number,
cars.car_name,
cars.price
FROM cars, (SELECT #rn := 0) var
) carsTable1;
Demo Here
Sample Input:
car_name price
car 1 100
car 2 200
car 3 300
Sample Output:
car_name price diff
car 1 100 100
car 2 200 NULL
car 3 300 100
Note: The price of car 2 is compared with the price of the rest of the cars. So the result shows null for car 2 since it's the reference car.
If I misunderstood your requirement then it must be : You want the price differences between the consecutive rows i.e. (No car,car1),(car1,car2), (car2,car3), (car3,car4)....
So in this case you can adopt the following query :
SOLUTION #2
SELECT
car_name,
cars.price,
CASE WHEN #currentPrice = 0 THEN NULL ELSE ABS(cars.price - #currentPrice) END AS diff,
#currentPrice := price
FROM cars ,(SELECT #currentPrice := 0) var
ORDER BY car_name
SQL FIDDLE BASED ON THIS QUERY
And if you want to omit the fourth column:
SELECT
t.car_name,
t.price,
t.diff
FROM
(
SELECT
car_name,
cars.price,
CASE WHEN #currentPrice = 0 THEN NULL ELSE (cars.price - #currentPrice) END AS diff,
#currentPrice := price
FROM cars ,(SELECT #currentPrice := 0) var
ORDER BY car_name ) t
SQL FIDDLE BASED ON THIS QUERY

Answer 3

Try this:-
CREATE TABLE #TempTable (rownum INT, price int, car_name VARCHAR(256));
INSERT INTO #TempTable (rownum, price, car_name)
SELECT
rownum = ROW_NUMBER() OVER (ORDER BY c.car_id),
c.price,
c.car_name
FROM car c;
SELECT
NEX.car_name + '-' + TT.car_name,
(nex.price - tt.price) AS Differences
FROM #TempTable TT
LEFT JOIN #TempTable prev ON prev.rownum = TT.rownum - 1
LEFT JOIN #TempTable nex ON nex.rownum = TT.rownum + 1;