Warning: Mysql_fetch_array(): Supplied Argument Is Not A Valid MySQL Result

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hi, well all of we looks always help from someone but we don't see what's the mistake take a look. while $sql used a select query and in $result you will see mysql_query($sql, $db); there is error. let make it easy using the $db before $sql like mysql_select_db($db);

hope you will get rid from the problem.

I am getting an error of No database selected
plz provide a solution

my code
?php
$con = mysql_connect(localhost," (!$con)
{
die('Could not connect: ' . mysql_error());
}
else echo"connected";



m * FROM Persons";

$result = mysql_query($sql) or die(mysql_error());

wh = mysql_fetch_array($result {
echo $row['FirstName'];
echo "<br />";
}
mysql_close();
?>

i am having same warning when run the page online and some time it run with no error message.
when i am runing it on localhost then error message never come.
plz tell me what may be the problem ant its possible solution also.
thanks in advance.

Basically this error is telling you that mysql could not find the data from the database. This is the problem:

you are telling the database:

Grab the data from the "titleid" table where the field "sid" matches whatever value $catid has in the pages table of your database.

So simply put, you are getting this error because not matches were found. Either because the table or field is empty or you may have the $catid variable as empty (false)

To resolve this problem, check to make sure your database has data that matches the $catid
https://www.wallpaperama.com/forums/warning-mysql-fetch-array-supplied-argument-is-not-a-valid-mysql-result-t78.html