SQL - Calculate the difference in number of orders by month - sql

Question

I am working on the orders table provided by this site, it has its own editor where you can test your SQL statements.
The order table looks like this
order_id
customer_id
order_date
1
7000
2016/04/18
2
5000
2016/04/18
3
8000
2016/04/19
4
4000
2016/04/20
5
NULL
2016/05/01
I want to get the difference in the number of orders for subsequent months.
To elaborate, the number of orders each month would be like this
SQL Statement
SELECT
MONTH(order_date) AS Month,
COUNT(MONTH(order_date)) AS Total_Orders
FROM
orders
GROUP BY
MONTH(order_date)
Result:
Month
Total_Orders
4
4
5
1
Now my goal is to get the difference in subsequent months which would be
Month
Total_Orders
Total_Orders_Diff
4
4
4 - Null = Null
5
1
1 - 4 = -3
My strategy was to self-join following this answer
This was my attempt
SELECT
MONTH(a.order_date),
COUNT(MONTH(a.order_date)),
COUNT(MONTH(b.order_date)) - COUNT(MONTH(a.order_date)) AS prev,
MONTH(b.order_date)
FROM
orders a
LEFT JOIN
orders b ON MONTH(a.order_date) = MONTH(b.order_date) - 1
GROUP BY
MONTH(a.order_date)
However, the result was just zeros (as shown below) which suggests that I am just subtracting from the same value rather than from the previous month (or subtracting from a null value)
MONTH(a.order_date)
COUNT(MONTH(a.order_date))
prev
MONTH(b.order_date)
4
4
0
NULL
5
1
0
NULL
Do you have any suggestions as to what I am doing wrong?

Answer 1

You have to use LAG window function in your SELECT statement.
LAG provides access to a row at a given physical offset that comes
before the current row.
So, this is what you need:
SELECT
MONTH(order_date) as Month,
COUNT(MONTH(order_date)) as Total_Orders,
COUNT(MONTH(order_date)) - (LAG (COUNT(MONTH(order_date))) OVER (ORDER BY (SELECT NULL))) AS Total_Orders_Diff
FROM orders
GROUP BY MONTH(order_date);
Here in an example on the SQL Fiddle: http://sqlfiddle.com/#!18/5ed75/1
Solution without using LAG window function:
WITH InitCTE AS
(
SELECT MONTH(order_date) AS Month,
COUNT(MONTH(order_date)) AS Total_Orders
FROM orders
GROUP BY MONTH(order_date)
)
SELECT InitCTE.Month, InitCTE.Total_Orders, R.Total_Orders_Diff
FROM InitCTE
OUTER APPLY (SELECT TOP 1 InitCTE.Total_Orders - CompareCTE.Total_Orders AS Total_Orders_Diff
FROM InitCTE AS CompareCTE
WHERE CompareCTE.Month < InitCTE.Month) R;

Answer 2

Something like the following should give you what you want - disclaimer, untested!
select *, Total_Orders - lag(Total_orders,1) over(order by Month) as Total_Orders_Diff
from (
select Month(order_date) as Month, Count(*) as Total_Orders
From orders
Group by Month(order_date)
)o