INSERT or UPDATE using IF-ELSE php MYSQL

DONOR
 donor-nic----username-----status
 111----------ali----------available
 222---------usman--------notavailable
 

another is

DONATION
 donation_id------donor_nic-----date---
 1----------------111----------2012/03/04
 2---------------111-----------2012/06/07
 3----------------111---------2012/07/08
 4----------------222---------2012/03/03
 

now i want to update the date if it is already exits corresponding to donor_nic
if date does not exists i want to insert that new date,.

i am using this query to fetching all dates according to corresponding donor_nic

SELECT donor.donor_nic, donation.lastdonationdate
 FROM donor
 JOIN donation ON donor.donor_nic = donation.donor_nic
 WHERE username = 'ali'
 GROUP BY donation.lastdonationdate
 

now this query works in phpmyadmin.. but when i use this in my page.. it does not match date.. i am matching date like this

   $sql= SELECT donor.donor_nic, donation.lastdonationdate
     FROM donor
     JOIN donation ON donor.donor_nic = donation.donor_nic
     WHERE username = 'ali'
     GROUP BY donation.lastdonationdate
 
 $res=mysql_query($sql, $con);
 
 if($res == $date)
 {
 echo "update";
 }
 else
 {
 echo "insert";
 }
 

but this thing didnt work.. may be i am making mistake in conditional (if-else) statment.. plz help me what should i do..
thanks in advance

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