Rank by calculated variable in MySQL - mysql

Question

The following table is for practice only. I will use the code on a much larger table.
SELECT *
FROM price_practice;
gives
id company dt price
'16', 'Amex', '2015-07-01', '5.00'
'17', 'Amex', '2015-07-02', '5.10'
'18', 'Amex', '2015-07-03', '5.00'
'19', 'Amex', '2015-07-06', '5.88'
'20', 'Amex', '2015-07-07', '4.21'
'21', 'Citi', '2015-07-01', '1.00'
'22', 'Citi', '2015-07-02', '1.10'
'23', 'Citi', '2015-07-03', '1.00'
'24', 'Citi', '2015-07-06', '0.88'
'25', 'Citi', '2015-07-07', '1.01'
'26', 'Amex', '2015-07-08', '5.23'
'27', 'Amex', '2015-07-09', '5.35'
'28', 'Amex', '2015-07-10', '5.55'
'29', 'Amex', '2015-07-13', '5.88'
'30', 'Amex', '2015-07-14', '6.01'
'31', 'Citi', '2015-07-08', '0.95'
'32', 'Citi', '2015-07-09', '0.83'
'33', 'Citi', '2015-07-10', '0.79'
'34', 'Citi', '2015-07-13', '0.72'
'35', 'Citi', '2015-07-14', '0.59'
The following snippet calculates the percentage change in price from one date to the next.
SELECT x.id, x.company, x.dt, x.price, (x.price - y.price)/y.price AS 'Change'
FROM
(
SELECT a.id AS aid, MAX(b.id) AS aPrevid
FROM price_practice a
INNER JOIN price_practice b
WHERE a.id > b.id
AND a.company = b.company
GROUP BY a.id
) Sub1
INNER JOIN price_practice x ON Sub1.aid = x.id
INNER JOIN price_practice y ON Sub1.aPrevid = y.id
ORDER BY x.id DESC
As intended, it returns
id company dt price Change
'35', 'Citi', '2015-07-14', '0.59', '-0.180556'
'34', 'Citi', '2015-07-13', '0.72', '-0.088608'
'33', 'Citi', '2015-07-10', '0.79', '-0.048193'
'32', 'Citi', '2015-07-09', '0.83', '-0.126316'
'31', 'Citi', '2015-07-08', '0.95', '-0.059406'
'30', 'Amex', '2015-07-14', '6.01', '0.022109'
'29', 'Amex', '2015-07-13', '5.88', '0.059459'
'28', 'Amex', '2015-07-10', '5.55', '0.037383'
'27', 'Amex', '2015-07-09', '5.35', '0.022945'
'26', 'Amex', '2015-07-08', '5.23', '0.242280'
'25', 'Citi', '2015-07-07', '1.01', '0.147727'
'24', 'Citi', '2015-07-06', '0.88', '-0.120000'
'23', 'Citi', '2015-07-03', '1.00', '-0.090909'
'22', 'Citi', '2015-07-02', '1.10', '0.100000'
'20', 'Amex', '2015-07-07', '4.21', '-0.284014'
'19', 'Amex', '2015-07-06', '5.88', '0.176000'
'18', 'Amex', '2015-07-03', '5.00', '-0.019608'
'17', 'Amex', '2015-07-02', '5.10', '0.020000'
The following snippet does something entirely different: it ranks observations by price for every company seperately.
SELECT (
CASE company
WHEN #curType
THEN #curRow := #curRow + 1
ELSE #curRow := 1 AND #curType := company END
) + 1 AS rank,
id,
company,
dt,
price
FROM price_practice,
(SELECT #curRow := 0, #curType := '') r
ORDER BY company DESC, price DESC;
As intended, it returns
rank id company dt price
'1', '22', 'Citi', '2015-07-02', '1.10'
'2', '25', 'Citi', '2015-07-07', '1.01'
'3', '23', 'Citi', '2015-07-03', '1.00'
'4', '21', 'Citi', '2015-07-01', '1.00'
'5', '31', 'Citi', '2015-07-08', '0.95'
'6', '24', 'Citi', '2015-07-06', '0.88'
'7', '32', 'Citi', '2015-07-09', '0.83'
'8', '33', 'Citi', '2015-07-10', '0.79'
'9', '34', 'Citi', '2015-07-13', '0.72'
'10', '35', 'Citi', '2015-07-14', '0.59'
'1', '30', 'Amex', '2015-07-14', '6.01'
'2', '19', 'Amex', '2015-07-06', '5.88'
'3', '29', 'Amex', '2015-07-13', '5.88'
'4', '28', 'Amex', '2015-07-10', '5.55'
'5', '27', 'Amex', '2015-07-09', '5.35'
'6', '26', 'Amex', '2015-07-08', '5.23'
'7', '17', 'Amex', '2015-07-02', '5.10'
'8', '18', 'Amex', '2015-07-03', '5.00'
'9', '16', 'Amex', '2015-07-01', '5.00'
'10', '20', 'Amex', '2015-07-07', '4.21'
The question is:
How do I rank observations by percentage change?
I imagine you can save the percentage change data in a new column and then rank it, but I suspect this is not the best method. I will do many similar calculations (eg weekly % change, variance etc), and I have around 3,000,000 observations, so the table would grow big quickly. If this is the only way to do it, I will, but I think combining the two snippets above to calculate percentage change and rank in one go would be better. Or what do you think?
As I'm sure you can tell from my question, I'm a beginner at MySQL. Any advise on how to proceed is appreciated!