How to Calculate Dates Ignoring the Year in SQL?

How to perform date calculations that ignore the year using SQL

Problem:

You want to select dates that have an anniversary within the next 14 days, but you want to do this by excluding the year.

Example Query:

SELECT *
FROM events
WHERE EXTRACT(month FROM "date") = 3
AND EXTRACT(day FROM "date") < EXTRACT(day FROM "date") + 14;

Challenge:

The above query doesn't work because it doesn't account for the wrap-around effect of months.

Solutions:

1. Advanced Version

This solution uses a custom SQL function and a multicolumn index for optimal performance.

CREATE OR REPLACE FUNCTION f_mmdd(date)
  RETURNS int LANGUAGE sql IMMUTABLE PARALLEL SAFE STRICT AS
'SELECT EXTRACT(month FROM )::int * 100 + EXTRACT(day FROM )::int';

CREATE INDEX event_mmdd_event_date_idx ON event(f_mmdd(event_date), event_date);

SELECT *
FROM event e
WHERE f_mmdd(e.event_date) BETWEEN f_mmdd(current_date)
                            AND f_mmdd(current_date + 14);

2. Table Function

This solution uses a PostgreSQL table function to handle the anniversary calculation.

CREATE OR REPLACE FUNCTION f_anniversary(_the_date date = current_date, _days int = 14)
  RETURNS SETOF event
  LANGUAGE plpgsql AS
$func$
DECLARE
   d  int := f_mmdd();
   d1 int := f_mmdd( +  - 1);  -- fix off-by-1 from upper bound
BEGIN
   IF d1 > d THEN
      RETURN QUERY
      SELECT *
      FROM   event e
      WHERE  f_mmdd(e.event_date) BETWEEN d AND d1
      ORDER  BY f_mmdd(e.event_date), e.event_date;

   ELSE  -- wrap around end of year
      RETURN QUERY
      SELECT *
      FROM   event e
      WHERE  f_mmdd(e.event_date) >= d OR
             f_mmdd(e.event_date) <= d1
      ORDER  BY (f_mmdd(e.event_date) >= d) DESC, f_mmdd(e.event_date), event_date;
      -- chronological across turn of the year
   END IF;
END
$func$;

SELECT * FROM f_anniversary();

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