prevent insert data to database if specific atribute already exist - php

Question

I want to prevent duplicate values into a database table from a form using PHP.
I have a table data on database that have atribute dataid(auto increment as primary key), data1, data2.
And I have a simple form like this
<h2>Enter your data</h2>
<form action="script.php" method="post">
Data 1:<input type="text" name="data1" /></p>
Data 2:<textarea name="data2"></textarea></p>
<input type="submit" name="submit" value="Add Data" />
</form>
It's script.php for inserting data to database
<?php
if(isset($_POST['submit']))
{
//connect to the database
$conn = mysql_connect('host', 'username', 'password', 'dbname') or die(mysql_error());
//insert results from the form input
$query = "INSERT INTO data(data1, data2) VALUES('$_POST[data1]', '$_POST[data2]')";
$result = mysql_query($conn, $query) or die(mysql_error());
}
?>
but it will insert any data from the form to database.
How can I prevent insert data to database if data1 already exist?

Answer 1

The solution to your problem is to make the column unique so the database takes care of enforcing this constraint. You can do this by creating a unique index or constraint:
alter table data add constraint unq_data_data1 unique (data1);
You can then use this in an insert to ignore duplicates by using on duplicate key update:
INSERT INTO data(data1, data2)
VALUES('$_POST[data1]', '$_POST[data2]')
ON DUPLICATE KEY UPDATE data1 = VALUES(data1);
The ON DUPLICATE KEY part doesn't really do anything. It is a no-op.
Also, you should parameterize your queries, so you are not subject to SQL injection and so the query plans can be cached. But that is another matter.

Answer 2

make column data1 unique
ALTER TABLE `data` ADD UNIQUE (`data1`);
change your query like this to ignore insert when data exist
$query = "INSERT IGNORE INTO data(data1, data2) VALUES('$_POST[data1]', '$_POST[data2]')";