sql查询年份差异取决于特定月份

Question

I am developing a book reservation system for a library. In the table of the Reservations there are the fields: id_reservation, id_user, id_book, datetime.

The id_user has a specific format eg: 2009897652 . The first four digits indicate the year that the user was registered in the system. All the users are registered in September (school library).

I want to count the reservations GROUP BY the years that the user is registered. So I have to find the difference (integer in years) between the date of today and the September of registration year.

Any help how to count the reservations GROUP BY the years that the users are registered?

eg.

total_reservations| users_registered_years
 200              | 1 
 220              | 2 
 320              | 3
 400              | 4

Thanks a lot!!

Answer 1

Why not just do it by the year?

select left(id_user, 4) as year, count(*)
from reservations 
group by left(id_user, 4)
order by year;

You can then just subtract the current year:

select year(now()) - left(id_user, 4) as diff, count(*)
from reservations 
group by left(id_user, 4)
order by diff;

I'm not sure if it worth fixing the results to make it right on any date, unless you plan on putting this query into an application. Then the logic is just to add an offset, like year(now()) - left(id_user, 4) + (case when month(now()) >= 10 then 1 else 0 end) .