php 'no database selected' but it is connected to the database - php

Question

I have checked other questions with the same title but can't fix this. I have connected to the database though because the login works and I can display the user's username. Here is the relevent code, the second part is what's causing the error, thanks:
<?php require 'connection.php' ?>
<?php
session_start();
?>
<content>
<?php
//Select all from the categories table and order by the category title in ascending order (ASC)
$sql = "SELECT * FROM categories ORDER BY category_title ASC";
//$res = the result of the above query or die if error
$res = mysql_query($sql) or die (mysql_error());
if (mysql_num_rows($res) > 0) {
} else {
"<p>No categories availible</p>";
}
?>
</content>

Answer 1

Here is an example, for the visual of it.
<?php
$dbname = 'so_gibberish';
$dbuser = 'GuySmiley';
$dbpass = 'MyPassword';
$dbhost = 'localhost';
$link = mysql_connect($dbhost, $dbuser, $dbpass) or die("Unable to Connect to '$dbhost'");
mysql_select_db($dbname) or die("Could not open the db '$dbname'");
$test_query = "SHOW TABLES FROM $dbname";
$result = mysql_query($test_query);
...
...
mysql_close($link);
However, go with PDO
It is less sickly, full of robustness, and safer to say the least for binding:
Again, just a visual.
<?php
$theNum=102; // get from user, this is hard-coded
try {
$dbh = new PDO('mysql:host=localhost;dbname=so_gibberish;charset=utf8', 'GuySmiley', 'MyPassword');
$dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$stmt = $dbh->prepare('SELECT id,smin,smax from ranges7
WHERE smin <= :theNum AND smax >= :theNum');
$stmt->bindParam(':theNum', $theNum, PDO::PARAM_INT);
$stmt->execute();
while ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
// one row we hope
$data = "id: ".$row['id'] . ", smin: " . $row['smin'] . ", smax: " . $row['smax'] . "\n";
print $data;
}
$stmt = null;
// PDO closes connection at end of script
} catch (PDOException $e) {
echo 'PDO Exception: ' . $e->getMessage();
exit();
}
?>