php - duplicate entry - php

Question

what should do if the entry are doubled?
<?php
require_once('auth.php');
session_start();
$exam = $_SESSION['exam'];
$subject_id = $_SESSION['exam'];
$_SESSION['sub'] = $subject_id;
$subject_title = $_POST['subject_title'];
$subject_description = $_POST['subject_description'];
$con = mysql_connect("localhost","root","");
if (!$con)
{
die('Could not connect: ' . mysql_error());
}
mysql_select_db('db_compre', $con);
$sql = "INSERT INTO examsubjectrecord_table(subject_id , subject_title ,
subject_description)
VALUES ('$subject_id','$subject_title', '$subject_description')";
if (!mysql_query($sql,$con))
{
die('Error: ' . mysql_error());
}
else
{
header("location: addsubject.php?exam=".$exam ."");
}
?>`
Notice: A session had already been started - ignoring session_start() in C:\xampp\htdocs\compre\admin\addsubjectacc.php on line 4
**Error: Duplicate entry '1' for key 'PRIMARY'**

Answer 1

It depends on yout application business-logic.
You can notify a user about a duplicated entry or silently update information with INSERT ... ON DUPLICATE KEY UPDATE ... SQL statement.

Answer 2

In your database you have a primary key of subject_id which cant have duplicates.
If you need to have duplicates in the subject_id column then you should add a column and set it as a primary key in your database. For example add another column unique_id and set it to auto_increment and as a primary key for row identification.

Answer 3

Basically, you'll first want to check if the value you're trying to insert into your primary key field already exists.
So if you primary key field is subject_id, you'd need to check if that already exists by doing a select query followed by PHP's mysql_num_rows function. For example:
$subject_id = 1337;
$check = mysql_query("SELECT `subject_id` FROM `examsubjectrecord_table` WHERE `subject_id`=" . $subject_id);
// See if anything was returned
if(mysql_num_rows($check) > 0) {
// We have something with this subject_id already!
echo "Cannot insert duplicate subject!";
} else {
// All clear, run your INSERT query here
}

Answer 4

Which column is your primary key? I'm going to assume that's subject_id. This needs to be unique for each row in your table. The easiest way to ensure this is to use AUTO_INCREMENT and then avoid inserting the subject_id at all. It will be assigned automatically.
If you need to find out what the ID of new subjects is, you can use mysql_insert_id.