Leetcode sql 1393. Capital Gain/Loss

Table: Stocks

Column Name	Type
stock_name	varchar
operation	enum
operation_day	int
price	int

(stock_name, day) is the primary key for this table.
The operation column is an ENUM of type ('Sell', 'Buy')
Each row of this table indicates that the stock which has stock_name had an operation on the day operation_day with the price.
It is guaranteed that each 'Sell' operation for a stock has a corresponding 'Buy' operation in a previous day.

question:
Write an SQL query to report the Capital gain/loss for each stock.
The capital gain/loss of a stock is total gain or loss after buying and selling the stock one or many times.
Return the result table in any order.

The query result format is in the following example:

Stocks table:

stock_name	operation	operation_day	price
Leetcode	Buy	1	1000
Corona Masks	Buy	2	10
Leetcode	Sell	5	9000
Handbags	Buy	17	30000
Corona Masks	Sell	3	1010
Corona Masks	Buy	4	1000
Corona Masks	Sell	5	500
Corona Masks	Buy	6	1000
Handbags	Sell	29	7000
Corona Masks	Sell	10	10000

Result table:

stock_name	capital_gain_loss
Corona Masks	9500
Leetcode	8000
Handbags	-23000

Leetcode stock was bought at day 1 for 1000 dollar and was sold at day 5 for 9000 dollar. Capital gain = 9000 - 1000 = 8000 dollar.
Handbags stock was bought at day 17 for 30000 dollar and was sold at day 29 for 7000 dollar. Capital loss = 7000 - 30000 = -23000 dollar.
Corona Masks stock was bought at day 1 for 10 dollar and was sold at day 3 for 1010 dollar. It was bought again at day 4 for 1000 dollar and was sold at day 5 for 500 dollar. At last, it was bought at day 6 for 1000$ and was sold at day 10 for 10000 dollar. Capital gain/loss is the sum of capital gains/losses for each ('Buy' --> 'Sell') operation = (1010 - 10) + (500 - 1000) + (10000 - 1000) = 1000 - 500 + 9000 = 9500 dollar.

select distinct stock_name,
sum(if(operation='Sell',price,0))-sum(if(operation='Buy',price,0)) as capital_gain_loss 
from stocks
group by stock_name

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/capital-gainloss