Insert data in table using drupal form alter submit I am currently work with a drupal's inbuilt form which is coming from it's system function.
Now I added a extra field with value in the form with Drupal's custommodule_form_deal_node_form_alter(). 
It is working perfectly. 
But my problem is How to insert the value in the database with the default form's value. As the form's has its own default submit function.  I know it may be done using custommodule_form_deal_node_form_alter_submit($form,$form_state)
But dont know how to merge my value with the default form's submit function.
Please help me.
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.
}