获取所有月份的列表和当月订购的产品数量，没有订单的月份应该为“0” - mysql

Question

I have a set of data that looks like this

Cust Id order date  Ordered Product
1         Jan 2           1
1         Jan 5           2
1         March 14        1      
1        September 9      1
1        December 12      2
2          Jan 5          1
2          Feb 13         2
3         March 12        2
3          April 5        3
3          June 10        2

and my output should look like this

Cust Id                     order Date                  Order product
  1                           Jan 31                           3
  1                            feb 29                          0
  1                            Mar31                           1
  1                           Apr 30                           0
  1                           May 31                           0
  1                           June 30                          0
  1                           July 31                          0
  1                           Aug 31                           0
  1                           Sept 30                          1
  1                           oct 31                           0
  1                           Nov 30                           0
  1                           Dec 31                           2

and I have got this far

1   January 31       3
1   March 31         1
1   September 30     1
1   December 31      2

and my code is

select customer_id, 
date_format(last_day(order_date), '%M %d') as new_months, 
sum(products_ordered) as total
from amazon_test
where customer_id =1
group by new_months, customer_id;

I currently stuck at the part where I need to have all the months and '0' as the output since no orders were made.

Answer 1

If you are running MySQL 8.0, one option is to use a recursive query to generate the months, and then bring the table with a left join :

with recursive months as (
    select customer_id, date_format(min(order_date), '%Y-%m-01') order_date, max(order_date) max_order_date 
    from amazon_test
    group by customer_id
    union all
    select customer_id, order_date + interval 1 month, max_order_date 
    from months
    where order_date  + interval 1 month < max_order_date
)
select 
    m.customer_id, 
    date_format(last_day(m.order_date), '%M %d') new_months,
    coalesce(sum(t.products_ordered), 0) ordered_products
from months m
left join amazon_test t 
    on  t.customer_id = m.customer_id
    and t.order_date >= m.order_date
    and t.order_date <  m.order_date + interval 1 month
where m.customer_id = 1
group by m.customer_id, m.order_date
order by m.customer_id, m.order_date

I phrased the query so it actually operates on all customer_id s - if you remove the where clause in the outer query, you do get the results for all customers. If you really want the results for only one customer, you can optimize the query by pushing the where filter to the anchor of the recusive query.

Demo on DB Fiddle :

customer_id | new_months   | ordered_products
----------: | :----------- | ---------------:
          1 | January 31   |                3
          1 | February 29  |                0
          1 | March 31     |                1
          1 | April 30     |                0
          1 | May 31       |                0
          1 | June 30      |                0
          1 | July 31      |                0
          1 | August 31    |                0
          1 | September 30 |                1
          1 | October 31   |                0
          1 | November 30  |                0
          1 | December 31  |                2