Custom database query to validate data

Question

I cant find any syntax for custom queries, use of $wpdb is new to me so can anyone help me solving this??

if(isset($_POST['submit']))
{
 global $wpdb;

$tablename='wp_form_subscribe';

$myrows = $wpdb->get_results( "SELECT * FROM wp_form_subscribe   where     email=$wpdb->email" );
 if($myrows==0)
 {
 $data=array(
 'name' => $_POST['fullname'], 
 'age' => $_POST['age'],
 'email' => $_POST['email']);


  $wpdb->insert( $tablename, $data);
  }
  else 
  {
  $status='User already subscribed';
  }
  $status='';
  }

Answer 1

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.