When reading a table from a database, use the structure in the Tables as the input to your map. By default, it will read all of the rows from the table. If you wish to select less than that, use the DatabaseSelect on the root element of the table (the element about the Row element).
If you wish to read multiple unrelated tables (in the same or different databases) as an input to a map, create an enclosing structure with an element for each table to read. And in each table element, inherit from the Tables structure and then specify the DatabaseSelect as usual.
If you wish to read from multiple tables related by a join, see the join handling section below.
In the above screenshot you can see that it is reading all rows from the EMPLOYEES table whose EMPLOYEE_ID is greater than or equal to 200. Those elements are then mapped to an XML structure for the output.