SQL JOIN (help needed)

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The following SQL produces an output of sales data only if a PayPal txn_id exists. I dont understand how to use the JOIN statement instead of the AND statement to make the output complete (with and without paypal.txn_id)
Any help would be appreciated. The code is part of a PHP that will generate sales output, that will lead to the import of sales data to Microsoft Accounting Pro.

$sqlproducts = mysql_query("SELECT  a.orders_id, 
                                    a.date_purchased, 
                                    a.delivery_state, 
                                    a.payment_method, 
                                    b.products_model,  
                                    b.products_name, 
                                    b.products_quantity, 
                                    b.products_price,
                                    c.txn_id
                           FROM     orders a, orders_products b, paypal c 
                           WHERE    a.date_purchased between '2009-01-01' and  '2009-05-01'
                           AND      a.orders_id = b.orders_id
                           AND      a.orders_id = c.order_id
                           AND      a.date_purchased between '2009-01-01' and  '2009-05-01' 
                           ORDER BY orders_id, products_name ASC");