To join more than one table, you would use a SQL Join within the Key Provider or any SQL statement.
For example, you can create an SQL statement on the Documents > Line Items tab that is the Key Provider and gets order item and inventory information in one statement :
SELECT O.ItemNumber, O.OrderQty, I.ItemDescription, I.ItemWeight FROM OrderItems O Left Outer Join Inventory I on I.ItemNo = O.ItemNumber WHERE O.OrderNumber = {Header.Document Key}
Another option is to create multiple SQL statements and specify how the result tables will be joined.
You can use these fields : Join Field ID and By SQL Field Name to enter multiple SQL statements and join them together. This enables you :
Notes
Join Field ID gets its values from the SQL Field List. The field you specify for Join Field ID should be a Select field from another SQL that you want to join to the current SQL. It does not have to be the key provider, but the SQL you are joining to has to eventually itself be joined to the Is Key Provider SQL.
By SQL Field Name gets its values from the Select defined in the SQL Select statement.