If you’re looking where Magento stores its invoices, first step was to locate table sales_order_invoice, but such table doesn’t exists.
Good starting point is always a Magento API, which shows you a little bit how to digg for information.
I’ve located api.xml file responsible for invoices API and then found Model which does all magic.
public function items($filters = null) – to get all invoices
public function info($invoiceIncrementId) – how to read invoice details
Next very important thing – Entities – this will tell you how all elements are structured and where located in mysql table
'invoice' => array(
'entity_model' => 'sales/order_invoice',
Invoices are store in sales_order_entity ( try
SELECT * FROM sales_order_entity WHERE entity_type_id = 16)
'invoice_item' => array(
'entity_model' => 'sales/order_invoice_item',
'attributes' => array(
'parent_id' => array(
That should be enough to see what’s going on