I used it as a basis to calculate the difference in weeks, days, hours and minutes between two Unix date stamps.

$diff = $end_date — $start_date;
$weeks = floor($diff/60/60/24/7);
echo "Weeks Difference = ". $weeks;
$diff -= $weeks ? $weeks * (60*60*24*7) : 0;
$days = floor($diff/60/60/24);
echo "Days Difference = ". $days;
$diff -= $days ? $days * (60*60*24) : $difference;
$hours = floor($difference/60/60);
echo "Hours Difference = ". $hours;
$difference -= $hours ? $hours * (60*60) : $difference;
$minutes = floor($difference/60);
echo "Minutes Difference = ". $minutes;

plz give answer as soon as possible

You mention "dual" so I will assume you are working with Oracle.

Let me think of a more graceful way, but you should be able to achieve what you need with case statements in Oracle's PL/SQL. E.g.,

SELECT
case when date2 is null then date 1
when date1 is null then date2
else date1-date2
end
FROM dual;

Or something like that, hope that gets you started on the right track.

eg. select date1-date2 from dual;

Here if date2 is null, how do we manage to calculate the difference ? Same if date1 contains a null value ?