Custom database query to validate data

Assuming that all of your sql column names are correct, something like this should work:

if(isset($_POST['submit'])){
    global $wpdb;
    $tablename= $wpdb . 'form_subscribe';
    $myrows = $wpdb->get_var( $wpdb->prepare("SELECT email FROM $tablename WHERE email=%s LIMIT 1", $_POST['email']));
    if(empty($myrows)){
        $data=array(
            'name' => $_POST['fullname'], 
            'age' => $_POST['age'],
            'email' => $_POST['email']
        );
        $wpdb->insert( $tablename, $data);
    }
    else {
        $status="User already subscribed";
    }
    $status="";
}

I changed $wpdb->email to $_POST['email'], I think that is what you meant there. Additionally, I added the $wpdb->prepare() function for sanitation and $wpdb->prefix for best practices.

Since you are doing a lookup to check for something existing (i.e. 1 existing record means the same thing as 1000 existing records for our purposes) I changed $wpdb->get_results() to $wpdb->get_var() and added a limit clause. This will make the query more efficient.

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