How to calculate number of leap years between two dates in Oracle? - sql

Question

I want to calculate the number of leap years between the hire date of the employee and the current date (on hr.employees table in Oracle SQL Developer). How to do this?

Answer 1

A leap year consists of 366 days. I assume a "leap year" between two dates consists of all the days from Jan 1 to Dec 31 of a year with Feb 29th.
Based on this understanding, there is a pretty simple solution.
Count the number of days between the Jan 1 of the year following the hire date and Jan 1 of the year of the end date.
Count the number of years between those two dates.
Subtract the difference between the days and the number of years * 365
Happily, built-in functions do most of the work.
This results in:
(trunc(sysdate, 'YYYY') -
trunc(hiredate + interval '1' year, 'YYYY') -
365 * (extract(year from sysdate) - extract(year from hiredate) - 1)
) as num_years
It is a little trickier to count leap days but that is not what the question is asking.

Answer 2

You can reuse the code Oracle has already written: just check if creating a leap day raises an exception:
SELECT TO_DATE('2016-02-29','YYYY-MM-DD') FROM DUAL;
29.02.2016
but
SELECT TO_DATE('2018-02-29','YYYY-MM-DD') FROM DUAL;
ORA-01839: date not valid for month specified
So you just have to count the exceptions:
CREATE OR REPLACE FUNCTION count_leap_years (p_from DATE, p_to DATE) RETURN NUMBER
IS
number_of_leap_days NUMBER := 0;
date_not_valid EXCEPTION;
PRAGMA EXCEPTION_INIT(date_not_valid, -1839);
BEGIN
FOR y IN EXTRACT(YEAR FROM p_from) .. EXTRACT(YEAR FROM p_to) LOOP
DECLARE
d DATE;
BEGIN
d := TO_DATE(to_char(y,'fm0000')||'-02-29', 'YYYY-MM-DD');
IF p_from < d AND d < p_to THEN
number_of_leap_days := number_of_leap_days + 1;
END IF;
EXCEPTION WHEN date_not_valid THEN
NULL;
END;
END LOOP;
RETURN number_of_leap_days;
END count_leap_years;
/

Answer 3

Something like this will allow you to select numbers, in this case, from 1 to 2999.
Select Rownum year
From dual
Connect By Rownum <= 2999
Something like this will allow you to check if a specific year is a leap year or not
CASE WHEN ((MOD(YEAR, 4) = 0 AND (MOD(YEAR, 100) <> 0)) OR MOD(year, 400) = 0) THEN 1 ELSE 0 END as isLeapYear
Now you just need to add the filtering and the sum of the leap years.
Select sum(isLeapYear)
from (
Select year, CASE WHEN ((MOD(YEAR, 4) = 0 AND (MOD(YEAR, 100) <> 0)) OR MOD(year, 400) = 0) THEN 1 ELSE 0 END as isLeapYear
FROM (
Select Rownum year
From dual
Connect By Rownum <= 2999
) a
Where a.year >= EXTRACT(YEAR FROM DATE %DateStart%) and a.year <= EXTRACT(YEAR FROM DATE %DateEnd%)
) b
This can be really improved in terms of performance, but like this I think its easier do understand what each step is doing and then you can decompose it into what you really want and expect.

Answer 4

try this:
CREATE OR REPLACE FUNCTION LEAP_YEARS(EMP_ID IN NUMBER)
RETURN NUMBER
IS
HIRE_YEAR NUMBER:=0;
SYS_YEAR NUMBER:=0;
NUMBER_LEAP_YEARS NUMBER:=0;
BEGIN
SELECT EXTRACT(YEAR FROM HIRE_DATE) INTO HIRE_YEAR
FROM EMPLOYEE
WHERE EMPLOYEE_ID = EMP_ID;
SELECT EXTRACT(YEAR FROM SYSDATE()) INTO SYS_YEAR
FROM DUAL;
FOR IDX IN HIRE_YEAR..SYS_YEAR LOOP
IF MOD(IDX,4)=0 THEN
NUMBER_LEAP_YEARS := NUMBER_LEAP_YEARS + 1;
END IF;
END LOOP;
RETURN NUMBER_LEAP_YEARS;
END;
I tested it on a toy example and it works. Maybe you need to improve it (hint: maybe it needs some exceptions and it assume that hire_date < sysdate).
Then you can use it as:
SELECT LEAP_YEARS(E.EMPLOYEE_ID) FROM EMPLOYEE E;