1. A method for graphically composing on a computer system a query of a preexisting independent relational database including a plurality of source tables, comprising steps of: constructing a graphically displayed network of interconnected nodes generically representing source tables and relational query operators, respectively, in response to input from a user, each table node including an output icon representing contents of said represented source table, and each operator node including an output icon representing a table containing the result of said represented relational query operator; and automatically generating a query corresponding to a designated node in the network.