Error occurred ... no database selected - php

Question

I'm trying to connect to a database, check a column for whether a value exists or not, then execute a function based on whether or not that value exists.
Here's my code.
$con = mysql_connect('localhost','root','','users');
$sql = "SELECT * FROM allUsers WHERE username = '".mysql_real_escape_string($_POST["username"]) . "'";
$result = mysql_query($sql,$con) or die("Error occurred in [$sql]: " . mysql_error());
$count = mysql_num_rows($result);
if ($count != 0){
echo "Username is already taken";
echo "$count";
mysql_close($con);
}
else{
createUser($_POST["name"],$_POST["username"],$_POST["password"],$_POST["email"]);
}
The thrown error is:
Error occurred in [SELECT * FROM allUsers WHERE username = 'Admin']: No database selected.
I'm almost entirely sure that it comes from the $result line, but haven't a clue as to why.
I feel like this is a simple solution, and I'm just missing something minor.
I'm very new to MySQL (today is my first day, actually), so please keep solutions as simple as possible.

Answer 1

You forgot to call mysql_select_db after connecting:
$con = mysql_connect('localhost','root','');
mysql_select_db('users', $con);
Unlike MySQLi or PDO, mysql_* libraries does not take database as argument on the connection string, however if you were to migrate to either MySQLi or PDO.
MySQLi:
$con = new mysqli('localhost', 'root', '', 'users');
PDO:
$con = new PDO('mysql:host=localhost;dbname=users', 'root', '');
In MySQLi your code would look like this:
<?php
$host = "localhost";
$user = "root";
$pass = "";
$database = "users";
$con = mysqli_connect($host,$user,$pass,$database);
if($con->connect_error)
die('Connect Error (' . mysqli_connect_errno() . ') '. mysqli_connect_error());
$stmt = $con->prepare("SELECT * FROM allUsers WHERE username = ? LIMIT 1");
$stmt->bind_param('s',$_POST["username"]);
if (!$stmt->execute())
die('Failed to excute with error ' . $con->error);
$stmt->store_result();
$count = $stmt->num_rows;
$stmt->close();
if ($count > 0)
{
echo "Username is already taken.";
}
else
{
createUser($_POST["name"],$_POST["username"],$_POST["password"],$_POST["email"]);
}

Answer 2

I think your error is quite obvious, you need to specify the database you want.
mysql_select_db("databaseName", $con);
With that taken care of, please, please don't use mysql_ libraries the are vulnerable to SQL injection and will soon be removed.