Insert data in table using drupal form alter submit
In your alter hook you can create custom submit handlers.
Ex. 

function custommodule_form_deal_node_form_alter() {
    if (isset($form['actions']['submit'])) {
        $form['actions']['submit']['#submit'][] = 'custom_form_submit';
      }
    }


then you implement the custom submit handlers.

function custom_form_submit(&amp;$form, &amp;$form_state) {
 // $form_state will have the form value which you can insert into database.
}