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How to use mysql_num_rows to check if username has been used?

Posted on 2009-07-03
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Last Modified: 2012-05-07
I think I'm missing something simple on the code below, but I can't find the problem.  This is for a registration form and I'm just trying to query the table (users) and see if the username has already been used.  If the username has already been taken then tell the user, if the username hasn't been taken register the user.  What am I missing here?
// check the username and make sure it hasn't been taken
$result = mysql_query ("SELECT username from users WHERE username = '$new_user_name'");
$num_rows = mysql_num_rows($result);
 
if ($num_rows = '1') {
	echo "That username is already in use, please select another username! <a href='add_user.php'>Try Again</a>";
}
if ($num_rows = '0') {
		 mysql_query ("INSERT INTO users (first_name, last_name, username, password, email, state_id, state)
		 VALUES ('$first_name', '$last_name', '$user_name', '$encrypted_password', '$email', '$state_id', '$state_name')");	
         echo 'Good news, the user was added!';
      }

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Question by:jbpeake
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3 Comments
 
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Expert Comment

by:BrianMM
ID: 24772787
Is it doing anything?

try:
// check the username and make sure it hasn't been taken
$result = mysql_query ("SELECT username from users WHERE username = '$new_user_name'");
 
if($result) {
	if(mysql_num_rows($result) > 0) {
		echo "That username is already in use, please select another username! <a href='add_user.php'>Try Again</a>";
	} else {
		// Insert...
		mysql_query ("INSERT INTO users (first_name, last_name, username, password, email, state_id, state)
                 VALUES ('$first_name', '$last_name', '$user_name', '$encrypted_password', '$email', '$state_id', '$state_name')");     
		echo 'Good news, the user was added!';
	}
} else {
	echo "Sorry problem with query:".mysql_error();
}

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Opalelement earned 500 total points
ID: 24772811
I would agree with the "if($var > 0) else" method, as it means there will always be a Yes or No result, even if there happens to be more than 1.

However, in your code I am guessing you always got both?

The problem is with these:
if ($num_rows = '1')
if ($num_rows = '0')

You need another = sign in both of those, like this:
if ($num_rows == '1')
if ($num_rows == '0')

From http://www.php5-tutorial.com/basics/if-statements/
As you can see, we are using two equal signs after each other - why is that? In PHP, and in many other programming languages, a single equal sign is an assignment operator. It's used to assign a value to e.g. a variable. Two equal signs are a comparison operator, that is, it's used to compare two values. So, in this case, we use two equal signs to compare the two items: $number and 42. Had we only used one equal sign, 42 would be assigned to $number, and no comparison would have been made.
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LVL 11

Expert Comment

by:BrianMM
ID: 24772847
yes... that is correct also... should have spotted that in original code :)

General practice of using if(mysql_num_rows($result) > 0) is 'safer' incase for any reason there is more than one.
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