Double DUPLICATE KEY UPDATE entry error - php

Question

I was trying to upload a csv file to update a table in my db, but also wanted to prevent duplication. Initially a single DUPLICATE worked but when i added the second condition it didn't work.
I got "Duplicate entry '1' for key PRIMARY". How can i achieve my aim.
thanks for your help.
$sql = "INSERT INTO biodata (student_number, fname, lname, course_name, level)
VALUES('$data[0]', '$data[1]', '$data[2]', '$data[3]', '$data[4]')
DUPLICATE KEY UPDATE student_number = student_number
AND course_name = course_name";
mysql_query($sql) or die (mysql_error());

Answer 1

Replace AND with comma , in the UPDATE part.
Use the VALUES() syntax to refer to the values passed.
Also, you don't need to update the Primary or Unique Key (that activated the ON DUPLICATE KEY UPDATE):
$sql = "
INSERT into biodata
(student_number, fname, lname, course_name, level)
VALUES
('$data[0]', '$data[1]', '$data[2]', '$data[3]', '$data[4]')
ON DUPLICATE KEY UPDATE
--- student_number = VALUES(student_number),
--- removed
fname = VALUES(fname),
lname = VALUES(lname),
course_name = VALUES(course_name),
level = VALUES(level)
" ;
What the above code does:
If the INSERT fails (there is already a student with this student_number you are trying to Insert), the UPDATE is activated and the 4 other fields are updated.
It's your choice which fields to update. If you want to update for example, only the course_name field, just keep that line and remove the rest.

Answer 2

I think you misunterstood the meaning of the
ON DUPLICATE KEY UPDATE
construct. IT does not define a condition for when a duplciate key occurs, but the solution if such an event hast happened. So you can modify the key in some way, that does not result in a duplicate.
The conditions for duplicates is defined via the indexes and (primary) keys within the table definition. So according to your code above, you would add a unique key over student_number and course_name in the table definition.
For more references take a look at the docu.