Get all the customers who placed their first order this month

problem

Schema
------

customer
    id
    username
    created_at

order
    id,
    customer_id
    created_at


Scenario
--------
    Get all customers who placed their first order this month irrespective of their registration date.

Query - A
---------
SELECT 
    COUNT(CASE WHEN DATE_FORMAT(order.created_at, '%y-%m') < '17-12' THEN order.id END) previous_order,
    COUNT(CASE WHEN DATE_FORMAT(order.created_at, '%y-%m') > '17-11' THEN order.id END) orders,
    customer.id
FROM 
    order
INNER JOIN customer ON order.customer_id=customer.id 
GROUP BY 
    customer.id
HAVING
    previous_order = 0
AND orders > 0

Query - B
---------
SELECT 
    customer.id,
    COUNT(order.id) requests
FROM
    order
INNER JOIN
    customer ON order.customer_id = customer.id
WHERE 
	customer.id NOT IN (select customer_id from order where DATE_FORMAT(order.created_at, '%y-%m') < '17-12')
GROUP BY 
	customer.id

Benchmarking (ms)
-----------
A        B
-----------
109  |  141
125  |  140
125  |  141
109  |  125
109  |  125