MySQL 8.0

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CREATE TABLE transactions (
  id INT PRIMARY KEY AUTO_INCREMENT,
  code VARCHAR(10),
  date DATETIME,
  mode ENUM('Buy', 'Sell', 'Count', 'Return'),
  quantity INT,
  price DECIMAL(10,2),
  price_currency ENUM('ARS', 'USD'),
  usd_to_ars DECIMAL(10,2),
  return_id INT NULL DEFAULT NULL
)
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("a", "20180101", 'Buy', 4, 10, 'ARS', 3.7);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("a", "20180102", 'Buy', 9, 8, 'ARS', 5.8);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("a", "20180103", 'Sell', -3, 0, 'USD', 0);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("b", "20180104", 'Buy', 5, 5, 'USD', 8.9);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("a", "20180105", 'Buy', 2, 7, 'USD', 3.4);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("b", "20180106", 'Buy', 1, 8, 'ARS', 9);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("a", "20180107", 'Sell', -8, 0, 'USD', 4.4);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars) VALUES ("a", "20180108", 'Buy', 9, 9, 'ARS', 3.2);
INSERT INTO transactions (code, date, mode, quantity, price, price_currency, usd_to_ars, return_id) VALUES ("a", "20180109", 'Return', 6, 2, 'ARS', 2, 2);
SELECT * FROM transactions ORDER BY date DESC


id
code
date
mode
quantity
price
price_currency
usd_to_ars
return_id


9
a
2018-01-09 00:00:00
Return
6
2.00
ARS
2.00
2

8
a
2018-01-08 00:00:00
Buy
9
9.00
ARS
3.20
null

7
a
2018-01-07 00:00:00
Sell
-8
0.00
USD
4.40
null

6
b
2018-01-06 00:00:00
Buy
1
8.00
ARS
9.00
null

5
a
2018-01-05 00:00:00
Buy
2
7.00
USD
3.40
null

4
b
2018-01-04 00:00:00
Buy
5
5.00
USD
8.90
null

3
a
2018-01-03 00:00:00
Sell
-3
0.00
USD
0.00
null

2
a
2018-01-02 00:00:00
Buy
9
8.00
ARS
5.80
null

1
a
2018-01-01 00:00:00
Buy
4
10.00
ARS
3.70
null


Select * FROM (SELECT
       id, date, code, mode, quantity, price, price_currency, usd_to_ars, return_id,
       @acm := @acm + quantity as stock,
       @avr := (@avr * (@acm - quantity) + if(quantity > 0, quantity * if(mode = "Return", @avr, if(price_currency = 'USD', price, price / usd_to_ars)), quantity * @avr)) / @acm as average_price_usd
FROM
    transactions t1,
    (SELECT @acm := 0) x,
    (SELECT @avr := 0) y) t2
ORDER BY date DESC


id
date
code
mode
quantity
price
price_currency
usd_to_ars
return_id
stock
average_price_usd


9
2018-01-09 00:00:00
a
Return
6
2.00
ARS
2.00
2
25
3.020949380787554

8
2018-01-08 00:00:00
a
Buy
9
9.00
ARS
3.20
null
19
3.020949380787554

7
2018-01-07 00:00:00
a
Sell
-8
0.00
USD
4.40
null
10
3.208553823496352

6
2018-01-06 00:00:00
b
Buy
1
8.00
ARS
9.00
null
18
3.208553823496352

5
2018-01-05 00:00:00
a
Buy
2
7.00
USD
3.40
null
17
3.3450047020026736

4
2018-01-04 00:00:00
b
Buy
5
5.00
USD
8.90
null
15
2.85767199560303

3
2018-01-03 00:00:00
a
Sell
-3
0.00
USD
0.00
null
10
1.7865079934045454

2
2018-01-02 00:00:00
a
Buy
9
8.00
ARS
5.80
null
13
1.7865079934045451

1
2018-01-01 00:00:00
a
Buy
4
10.00
ARS
3.70
null
4
2.7027027027027026

id	code	date	mode	quantity	price	price_currency	usd_to_ars	return_id
9	a	2018-01-09 00:00:00	Return	6	2.00	ARS	2.00	2
8	a	2018-01-08 00:00:00	Buy	9	9.00	ARS	3.20	null
7	a	2018-01-07 00:00:00	Sell	-8	0.00	USD	4.40	null
6	b	2018-01-06 00:00:00	Buy	1	8.00	ARS	9.00	null
5	a	2018-01-05 00:00:00	Buy	2	7.00	USD	3.40	null
4	b	2018-01-04 00:00:00	Buy	5	5.00	USD	8.90	null
3	a	2018-01-03 00:00:00	Sell	-3	0.00	USD	0.00	null
2	a	2018-01-02 00:00:00	Buy	9	8.00	ARS	5.80	null
1	a	2018-01-01 00:00:00	Buy	4	10.00	ARS	3.70	null