In cases where two or more database tables can be joined by more than one relationship or criterion on the row values of selected columns, to produce what is effectively a wider table, there may result an ambiguity of which join should be used. The invention provides a means of join disambiguation that is rule-driven. The user is relieved of understanding the details of how the disambiguation is being done. The system described in this invention makes the choice of which join to use based on clear transformation rules which relate to folders of the tables and their relationships.