Check if there is data in table. Is my code Correct?

I have a table called “myguestbook”
Using $wpdb, how do I check if “jessy”
is already in the database under the table of “myguestbook”..

Here’s what i came up with…….

$query = "SELECT * FROM myguestbook"; 

// execute query
$result = mysql_query($query) or die ("Error in query: $query. ".mysql_error()); 

// see if any rows were returned
 if (mysql_num_rows($result) > 0) {
    // yes
    // print them one after another
    echo "<table cellpadding=10 border=1>";
    while($row = mysql_fetch_row($result)) {
        echo "<tr>";
        echo "<td>"."<center>".$row[0]."</center>"."</td>";
        echo "<td>" . $row."</td>";
        echo "<td>".$row."</td>";
        echo "</tr>";
    } 

}
else {
    // no
    // print status message
    echo "No rows found!";
}

That displays the names inside of the table.. Now the question is.. How can I get the results a IF statement..
Example

<?php if ($row(0) = $current_user->logged_in) {
//do something...
} ?>

In other words, im trying to check the current user who is logged in if they are in the database, If they are NOT in the database then //do something.. I know this is pretty simple.. atleast I would asume it is..

[ Please do not bump, it’s not permitted here. ]