How do you do date math that ignores the year?

Question

I am trying to select dates that have an anniversary in the next 14 days. How can I select based on dates excluding the year? I have tried something like the following.

Answer 1

I don't work with postgresql so I googled it's date functions and found this: http://www.postgresql.org/docs/current/static/functions-datetime.html

If I read it correctly, looking for events in the next 14 days is as simple as:

 where mydatefield >= current_date
 and mydatefield < current_date + integer '14'

Of course I might not be reading it correctly.

Answer 2

You can generate a virtual table of anniversaries, and select from it.

with anniversaries as (
  select event_date, 
         (event_date + (n || ' years')::interval)::date anniversary
  from events, generate_series(1,10) n
)
select event_date, anniversary
from anniversaries
where anniversary between current_date and current_date + interval '14' day
order by event_date, anniversary

The call to generate_series(1,10) has the effect of generating 10 years of anniversaries for each event_date. I wouldn't use the literal value 10 in production. Instead, I'd either calculate the right number of years to use in a subquery, or I'd use a large literal like 100.

You'll want to adjust the WHERE clause to fit your application.

If you have a performance problem with the virtual table (when you have a lot of rows in "events"), replace the common table expression with a base table having the identical structure. Storing anniversaries in a base table makes their values obvious (especially for, say, Feb 29 anniversaries), and queries on such a table can use an index. Querying an anniversary table of half a million rows using just the SELECT statement above takes 25ms on my desktop.