Derby-style table functions require a java class and table definition ahead of time.

unityJDBC Needs to generate XML files with table details/stats

SQL Server Linked Server uses OLEDB Drivers to access external systems (sp_addlinkedserver) MSDN Article

Oracle uses Heterogeneous Services & Generic Connectivity to connect to ODBC sources, although it seems finicky, seems unsupported on 64-bit, and sometimes requires service/listener configuration.

9075-9

SQL/MED

ISO/IEC 9075-9 Management of External Data (SQL/MED)

table expressions (postgressql)

table functions

cross database join

external tables