Building a report using multiple tables:

If using more than one table the tables must be related somehow. You need to understand your table structures when using more than one table in your report.

As an example in this demo the proper related tables could be:
(categories,products), (customers,orders), (orders,order_details), (order_details,products), (orders,shippers) and (products,suppliers).

In this demo you could select the related tables 'orders' & 'order_details'.
At step_3 select the tables orders & order_details
Next at table relations: Left table Order_Details & Left Field = OrderID
                                   Right table Orders & Right Field = OrderID

This is because the tables and the fields orders.orderid = orders_details.orderid , orderid is a primary key in orders table and foreign key in order_details table.

Note that if you select tables that are not related the SQL query will always work even with incorrect relations. It would be impossible to validate the relationships since everyone's db structure is different and everyone may look for different type reports. For example in the demo here if you define a relation as orders.orderid = orderdetails.unitprice , the sql engine will work yet the result will be the orders which carry the same value of unit price in the order details table or produce an empty report if there are no matches. You must define correct relationships for correct results.

To select more than one table for your report ** Hold the ctrl button to select additional table(s).