MySql how to compare this years month to last years month? - mysql

Question

How do I find the department numbers who have produced more than 20% more sales in them this February (as in this year so would use YEAR(CURDATE()) rather than saying "2012") compared to last February as in the year before. The result table should have only one column which contains the department numbers that have produced 20% more sales.
If the febraury last year has no sales for "department 2" but sales for this febraury, then 0 * 1.2= 0 so the results would not show "department 2". If the february last year for "department 3" sold one item, and the february this year sold two items, this would be a 1.5 increase which is more than 1.2 so the results should show "department 3". If there were only 3 departments, at most, there should be only 3 rows.
Department 2 does not appear because there was 3 sales last february and one sale this february. Department 4 does not appear because there was no sale last february so in terms of maths 0 * 1.2 = 0 so Department 4 would not appear despite there being sales this febraury.
Thank you in advance, and hope this is detailed enough.

Answer 1

Try using having as below:
select departement_no
from department
group by departement_no
having sum(case date_format(date, '%m%Y')
when CONCAT('02',YEAR(CURDATE())) then 1 else 0 end)
>
1.2 * sum(case date_form(date, '%m%Y')
when CONCAT('02',YEAR(DATE_SUB(CURDATE(),INTERVAL 1 Year)))
then 1 else 0 end)
AND
sum(case date_form(date, '%m%Y')
when CONCAT('02',YEAR(DATE_SUB(CURDATE(),INTERVAL 1 Year)))
then 1 else 0 end) > 0

Answer 2

This uses MAKEDATE() function to craete the first day of a year and then adds the appropriate month intervals to calculate the start of the (February) month and the start date of the next month.
Replace ? with 1 for January, 2 for February, etc.:
SELECT
this_year.departement_no
FROM
( SELECT departement_no, COUNT(*) AS num_sales
FROM department
WHERE date >= MAKEDATE( YEAR(NOW()), 1) + INTERVAL (?-1) MONTH
AND date < MAKEDATE( YEAR(NOW()), 1) + INTERVAL (?) MONTH
GROUP BY departement_no
) this_year
LEFT JOIN
( SELECT departement_no, COUNT(*) AS num_sales
FROM department
WHERE date >= MAKEDATE( YEAR(NOW())-1, 1) + INTERVAL (?-1) MONTH
AND date < MAKEDATE( YEAR(NOW())-1, 1) + INTERVAL (?) MONTH
GROUP BY departement_no
) last_year
ON last_year.departement_no = this_year.departement_no
WHERE
this_year.num_sales > 1.2 * COALESCE(last_year.num_sales, 0) ;
If you want (as your rather strange requirements) to not show departments that have sales this February and had no sales last year, remove the COALESCE() function. You can also change the LEFT join to INNER join:
WHERE
this_year.num_sales > 1.2 * last_year.num_sales ;