MySQL Num行问题

Question

I run this query to check if cat_name already exists it the mysql database... but it's show a warning messag.

Warning: mysql_num_rows() expects parameter 1 to be resource, boolean given in C:\\xampp\\htdocs......"

<?php
include("db.php");

if(isset($_POST['cat_name']))
{
$cat_name = mysql_real_escape_string(htmlentities(trim($_POST['cat_name'])));   
$err = array();

$ch = mysql_query("SELECT cat_name FROM categories WHERE cat_name = '$cat_name' ");
$num = mysql_num_rows($ch);

if(empty($cat_name))
$err[] = "Category field empty";
    elseif(is_numeric($cat_name))
    $err[] = "Category name should be string, ex: category name";
        elseif($num > 0)
        $err[] = "Category name already exits, please choose another name";
            else
            {
                if(strlen($cat_name) < 3)
                $err[] = "Category name at least 3 or more    
characters";    
            }

if(!empty($err))
{
    foreach($err as $er)
    {
        echo "<font color=red>$er.</font>";
    }   
}
else
{
    $sql_insert = mysql_query("INSERT INTO categoires VALUES('',  
'$cat_name')"); 

    if($sql_insert)
    {
        echo "Successfully inserted your category name";    
    }
    else
    {
        echo "Something is wrong to add your cateogory name";
        mysql_error();  
    }

}


}
?>

Any idea?

Answer 1

Your query is failing (and returning false ). Check mysql_error or the mysql/php error logs.

Answer 2

From the docs:

For SELECT, SHOW, DESCRIBE, EXPLAIN and other statements returning resultset, mysql_query() returns a resource on success, or FALSE on error.

mysql_query() will also fail and return FALSE if the user does not have permission to access the table(s) referenced by the query.

My best guess is that you have an error in your query.