Checking for duplicates in database table before inserting - php

Question

I need to create a form that gathers a first name, last name, and an email address. I have created a table in MySQL called guestbook. The table looks something like the following:
CREATE TABLE guestbook (
id int unsigned NOT NULL AUTO_INCREMENT,
firstName varchar(50) NOT NULL,
lastName varchar(50) NOT NULL,
email varchar(250) NOT NULL,
status int NOT NULL,
sort int NOT NULL
);
And here is my code thus far:
<?php
Global $Conn;
$Conn = new mysqli("localhost","151_millet","2gZMXYGC","GUESTBOOK");
if(!$Conn) {
$ErrorMsg = "Couldn't connect to the database";
}
$FName = $_POST["fname"];
$LName = $_POST["lname"];
$Email = $_POST["email"];
// The series of ifs below tests to see if each field is blank
// If it is blank it will output an error message for each that is blank
if($FName == "") {
$ErrorMsg .= "First Name Was Left Blank<br>";
}
if($LName == "") {
$ErrorMsg .= "Last Name Field Was Left Blank.<br>";
}
if($Email == "") {
$ErrorMsg .="Email field was left blank.<br>";
}
I have no idea how to check for duplicates in the email field. Any help would be appreciated. I think I need to come up with a way to test $_POST against whats already in the db.

Answer 1

Alrighty, now we've cleaned up this question a bit. Let's get down to business.
You want to check for duplicates in your database. There are a couple of things you need to do for this. Let's try to keep this as simple as possible while we're working on it too.
First off, it's good to see you're using MySQLi and not the deprecated mysql_* functions. But we can handle MySQLi in a more object oriented manner. Also, using globals is frowned upon (and is actually not necessary, even in your current code!)
One other thing to note is that there are slightly different styles of code used in modern PHP, like camel case, and how you format if statements. If you're interested in learning PHP, then you should take a look at the PHP-FIG PSRs.
<?php
$conn = new mysqli("localhost", "151_millet", "2gZMXYGC", "GUESTBOOK");
if ($conn->connect_errno) {
// This is an error that will stop us from continuing, so assigning
// the error message to a string, doesn't really help us in this case
// The application NEEDS to stop
throw new RuntimeException("Unable to connect to MySQL database.");
}
$firstName = $_POST["fname"];
$lastName = $_POST["lname"];
$email = $_POST["email"];
if (!$firstName || !$lastName || !$email) {
echo "Please make sure to fill in all of your details.";
// You may want to handle this differently, this is just to keep things
// Very simple
exit;
}
$query = "SELECT COUNT(1) FROM guestbook WHERE email = ?";
$stmt = $conn->prepare($query);
$count = 0;
if ($stmt) {
$stmt->bind_param("s", $email);
$stmt->execute();
$stmt->store_result();
$count = $stmt->num_rows;
$stmt->close();
}
if ($count > 0) {
echo "A user with that email address already exists.";
exit;
}
// Do other stuff
That should help you out with what you need to do to check if an email address already exists. But you should also enforce this in your database by adding a unique key to the email column. That will mean that even if your code fails and would allow a duplicate email address entry, your database won't.