Sub-queries, self-joins or View-Joins? - mysql

Question

I'm new here, so I hope I'll do everything right.
I'm trying to get a table on MySQL where I would get the sum of orders from a certain productline per month, and then comparing it with the orders from the same month in the previous year, to calculate the delta.
For that i need to group by each orders with the productline and month, and also month of last year. Of course, sometimes, there are no orders in certain months for certain productlines.
The tables columns would look like this :
ProductLine - MONTH(date) - SUM(ORDERS) - LAST_YEAR-MONTH(date) - LAST_YEAR_SUM(ORDERS) - DELTA
I can get current year table and last year table separately in views, but I can't effectively join them without messing the numbers, maybe because of the group-by clauses
Here what I tried (I seriously tried a 100 possibilities) :
Creating two views and left join the last-year table
Creating a subquery for the LAST_YEAR orders
Here is an example of what I tried, (forget about the delta, that part is the easy one) :
SELECT p.productLine pL
, MONTH(o.orderDate) oD
, SUM(od.quantityOrdered)
, MONTH(date_sub(o.orderDate, interval 1 year)) previous_year
, (SELECT SUM(orderdetails.quantityOrdered)
FROM orderdetails
LEFT
JOIN orders
ON orderdetails.orderNumber = orders.orderNumber
LEFT
JOIN products
on products.productCode = orderdetails.productCode
WHERE orderDate in (previous_year)
) previous_order
FROM products p
JOIN orderdetails od
ON p.productCode = od.productCode
JOIN orders o
ON od.orderNumber = o.orderNumber
GROUP
BY pL
, oD
, previous_year
ORDER
BY oD DESC

Answer 1

If you are running MySQL 8.0, you can use window functions and a range specification:
select
p.productline,
date_format(o.orderdate, '%Y-%m-01') as ordermonth
sum(od.quantityordered) quantityordered,
max(sum(od.quantityordered)) over(
partition by productline
order by date_format(o.orderdate, '%Y-%m-01')
range between interval 1 year preceding and interval 1 year preceding
) previousquantityordered
from products as p
join orderdetails as od on p.productcode = od.productcode
join orders as o on od.ordernumber = o.ordernumber
group by p.productline, ordermonth
order by ordermonth desc

Answer 2

For simplified comparison based on a calendar year comparison, I would start Jan 1 of the prior year to now of current year. Doing this, you would get Jan 2019/2020, Feb 2019/2020, etc to current month. Since Oct, Nov, Dec have not happened yet, you could add in an "and month( order date ) is LESS than the current month we are in" since it did not happen yet this year to compare against
select o.*
from orders o
where o.orderDate >= '2019-01-01'
AND month( o.orderDate ) < month( curdate())
Now that the underlying record base is simplified, lets try to get your details.
To differentiate between last year and current, I am doing a conditional SUM(). So within the sum is a CASE/WHEN test. If the year of the order is same as current year, sum its qty, otherwise 0. Likewise, if less than current year, put into the last year qty bucket total. This way, you get all product lines, even those that may have sold last year and not this and vice-versa too. This would result in zero values in those respective columns. But obviously below is a much more simplified query to follow.
select
p.productLine,
MONTH(o.orderDate) OrderMonth,
SUM( case when year( o.orderDate ) = year( curdate())
then od.quantityOrdered else 0 end ) CurYearQty,
SUM( case when year( o.orderDate ) < year( curdate())
then od.quantityOrdered else 0 end ) LastYearQty
from
orders o
JOIN orderdetails od
on o.orderNumber = od.orderNumber
JOIN products p
on od.productCode = p.productCode
where
o.orderDate >= '2019-01-01'
AND month( o.orderDate ) < month( curdate())
group by
MONTH(o.orderDate),
p.productLine
order by
MONTH(o.orderDate),
p.productLine