if not exist insert

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Hi, I need to insert data into a table but only if it doesnt already exist.

 

I have tried numerous ways including counting rows and nothing works so help desperately needed here. ???

 

I have a form that submits the information to this page and then redirects back to the form page with the $errorMessage but for the life of me, it keeps on duplicating the data.

$query = fetch("SELECT * FROM avatar_blank WHERE `user_name` = '$userid' AND `avatar_name = 'default'");

if($query[avatar_name] == "default"){

$errorMessage = "You+already+have+that+avatar!";


header("Location: avatar.php?&error=$errorMessage");

}

else
mysql_query("INSERT INTO avatar_blank (user_name , avatar_url, avatar_name) VALUES ('$userid','$avatar_url','$avatar_name')");

$errorMessage = "You+now+have+a+$avatar_name+avatar!";


header("Location: avatar.php?&error=$errorMessage");


?>

 

Any help is appreciated

 

Here is another example Ive tried without all the values added

 

$query = mysql_query("SELECT * FROM avatar_blank WHERE `user_name` = '$userid' AND `avatar_name = ''");
$num = mysql_numrows($query);
if($num < 1){

INSERT INTO `avatar_blank` ( `user_name` , `avatar_url` , `avatar_name` )
VALUES (
'$userid', '', ''
);

}

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Seems no1 really knows the answer to this one.

 

I've googled it and none of it makes much sense