Question

I am getting the error: Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource in.... on line ..

Please can you tell me what I am doing wrong to cause this. I don’t believe that I am.

        if (mysql_num_rows(mysql_query("SELECT * FROM Likes WHERE `postID` = '$postID' AND `userID` = '$accountID'")) < 1) {
            exit("Cant like a post twice");
        }

Answer 1

you can also simplify your sql by selecting the count directly from the database, which is more efficient then selecting all the rows, and then calculating the count

$res = mysql_query("SELECT COUNT(*) FROM Likes WHERE 
                      `postID` = '$postID' AND `userID` = '$accountID'");

// check for mysql errors
if (mysql_error()) {
    die(mysql_error());
}

list($count) = mysql_fetch_row($res);