如何计算每个月每种客户类型的购买价值？

Question

I have two tables.

Customers

customer_id	customer_type
1	student
2	blue

Orders

order_id	customer_id	purchase_value	date
001	1	500	2021-06-03
002	2	600	2021-03-01

I want to make a query to show monthly total purchase from each customer type, which is expected to be like this:

month	student	blue
3	0	600
6	500	0

I've tried to use subquery like this:

SELECT 
month(order_date) as month, 
(SELECT sum(order_value)  from orders JOIN customers
    ON customers.customer_id = orders.customer_id
    WHERE customers.customer_type LIKE 'Business'
    GROUP BY month, customers.customer_type) as personal
from orders
JOIN customers
ON customers.customer_id = orders.customer_id

But still fail, I want to try to use subquery inside JOIN but kinda confuse. Do you have any solution to this problem?

Answer 1

Use conditional aggregation:

SELECT MONTH(o.order_date) as month, 
       SUM(CASE WHEN c.customer_type = 'student' THEN order_value END) as student,
       SUM(CASE WHEN c.customer_type = 'blue' THEN order_value END) as blue
FROM orders o JOIN
     customers c
     ON c.customer_id = o.customer_id
GROUP BY MONTH(o.order_date);

A subquery is unnecessary. Note that using month() without taking the year to account often produces invalid results.