sql - preventing duplicate row insertion in php/mysql

Question

how do you prevent the entry of duplicate records in php mysql? validating input before insertion to the table.

i am currently building a user registration section for my website but want to present them with an error if a duplicate username or email address is found in the database.

EDIT
This is what I have to far to insert rows... how would I implement both these answers here:

  <?php
        if (array_key_exists('confirm',$_POST)) {

                $Cuser = $_POST['Cuser']; 
                $Cemail = $_POST['Cemail']; 
                $Cpassword = $_POST['Cpassword'];
                $md5password = md5($_POST['Cpassword']);

                mysql_query("INSERT INTO tbl_users (`username`,`email`,`password`)
VALUES ('$Cuser', '$Cemail', '$md5password')"); 

                echo "Thank you. The user <b>".$Cuser."&nbsp;(".$Cemail.")</b> has been successfully added to the database.<p>

                <a href='myaccount.php' class='form'>Return</a>";       

                exit;
        }
        ?>

I'm guessing that I could implement it all in an if...else statement?

Answer 1

Define unique constraints for the username and email columns:

ALTER TABLE your_table ADD CONSTRAINT uk_username UNIQUE (username)
ALTER TABLE your_table ADD CONSTRAINT uk_email UNIQUE (email)

If the value attempting to be insertered or updated already exists in the table, MySQL will return an error stating the query violates the appropriate unique constraint (possibly both). It's up to you to setup PHP to handle this gracefully.