With the growing popularity and prominence of spatial database types in commercial databases there is an increasing amount of spatial data that exists directly in a database. At ArcGIS 10 the team wanted to provide ArcGIS users with more open access to their geographic information. To address this, Query Layers functionality was added to give users direct access to this information without requiring them to export the information into a geodatabase.
Query Layers from a functional standpoint are very simple: write a query, get a layer. A Query Layer is just a layer or stand alone table in ArcGIS that is defined by a SQL query. Any valid SQL can be used to generate the query as long as the result set returned by the query conforms to ArcGIS data modeling standards (standards such as one shape field, one spatial reference, etc…). The Query Layers functionality supports access to any of the databases supported by ArcGIS (Oracle, SQL Server, PostgreSQL, DB2 and Informix) and any of the spatial types available for those database platforms.
The bottom line is incorporating geographic information into GIS projects is now quick and simple, independent of where that information is stored. For more information on Query Layers, take a look at: