In the UR approach, users need only specify columns of interest, and the interface software infers the underlying joins for the corresponding query. Most astronomical databases are single-table catalogs describing sky objects and their properties. Each catalog row corresponds to an object and each column refers to a property of the object. Many user interfaces are designed to display the data of a single astronomical table. The query mechanism for such interfaces is straightforward because there are no table joins to calculate.
Some applications require that more than one table be queried at a time.
For instance, a user interface that cross-correlates more than one table must be able to query multiple tables. Also, a user interface requiring more than one table to describe catalog objects must be able to query multiple tables. In either case, the mechanism that produces the query is complicated. Most solutions require that predefined queries be embedded in the screen definitions or in the interface code. Unfortunately, this approach requires that screen designers be database domain experts.