How to calculate percent increase/decrease using previous value - mysql

Question

I have a query that returns total value of sales grouped by month. I would like to add a column that returns the growth/decrease in percentage comparing with previous rows.
I have tried already to use CREATE TEMPORARY TABLE, creating a variable #var :=, but nothing is really working. Ideally my result would be:
Month | Sales | Perc
1 | 100 | 0
2 | 150 | 50
3 | 100 | -33.33
The calculation to be done is: ((actual_value - previous_value) / previous_value) *100
About the tables, I working with sales, products, sales_items, so my query to retrieve the total amount of sales grouped by month is:
SELECT
MONTH(s.sale_date) AS Month,
SUM(p.retail_price * si.quantity) AS Sales_Amount
FROM sales s
INNER JOIN sales_items si ON s.sale_id = si.sale_id
INNER JOIN products p ON si.product_id = p.product_id
WHERE YEAR(s.sale_date) = '2018'
GROUP BY month
ORDER BY month;
As I'm using SUM to summarise the items of the sales and aggregating that into each sale_id, and after grouping by month, it seems complicated to use that value into a new column, so it would be great if someone has some idea how to do it.

Answer 1

In MySQL 8.0, you can turn the aggregate query to a subquery and use window function LAG to access the value of the previous month, like:
SELECT
Sales_Month,
Sales_Amount,
( Sales_Amount - LAG(Sales_Amount) OVER(ORDER BY Sales_Month) )
/LAG(Sales_Amount) OVER(ORDER BY Sales_Month) * 100 AS Percent_Increase
FROM (
SELECT
MONTH(s.sale_date) AS Sales_Month,
SUM(p.retail_price * si.quantity) AS Sales_Amount
FROM sales s
INNER JOIN sales_items si ON s.sale_id = si.sale_id
INNER JOIN products p ON si.product_id = p.product_id
WHERE YEAR(s.sale_date) = '2018'
GROUP BY Sales_Month
) x
ORDER BY Sales_Month

Answer 2

You can add a new column "PrevMonthSales". To do that you have to join your query with itself :
SELECT ...
FROM
(...your_query...) s1
LEFT JOIN (...your_query...) s2 ON s2.Month = s1.Month - 1
In your case:
SELECT s1.Month, s1.Sales_Amount,
COALESCE(s2.Sales_Amount, 0) "PrevMonthSales",
CASE
WHEN s1.Sales_Amount = 0 THEN 0
ELSE (s1.Sales_Amount - COALESCE(s2.Sales_Amount, 0)) / s1.Sales_Amount
END "Percent"
FROM
(...your_query...) s1
LEFT JOIN (...your_query...) s2 ON s2.Month = s1.Month - 1

Answer 3

Make below a temp table called month_sales
SELECT
MONTH(s.sale_date) AS Month,
SUM(p.retail_price * si.quantity) AS Sales_Amount
FROM sales s
INNER JOIN sales_items si ON s.sale_id = si.sale_id
INNER JOIN products p ON si.product_id = p.product_id
WHERE YEAR(s.sale_date) = '2018'
GROUP BY month
ORDER BY month;
Query to find month over month difference is :
SELECT *,
case
when m2.Sales_Amount is null then 0
else (m2.Sales_Amount - m1.Sales_Amount)/m1.Sales_Amount
end as Perc
FROM month_sales m1
left Join month_sales m2
WHERE m1.month = m2.month - 1