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The actual SQL that may be used varies somewhat between databases. For example, different databases provide different support for outer joins. The java.sql.DatabaseMetaData interface provides a number of methods that can be used to determine exactly which SQL features are supported by a particular database. Similarly, the syntax for a number of SQL features may vary between databases and can also be discovered from java.sql.DatabaseMetaData. However, in order to pass JDBC conformance tests a driver must support at least ANSI SQL-2 Entry Level syntax and semantics plus support for the SQL extensions listed in Section 11.
Finally, some fundamental properties, such as transaction isolation levels, vary between databases. The default properties of the current database and the range of properties it supports can also be obtained from DatabaseMetaData.
Database vendors may add additional functionality by adding new subtypes of existing JDBC types that provide additional methods. Thus the Foobah corporation might define a new Java type foobah.sql.FooBahStatement that inherits from the standard java.sql.Statement type but adds some new functionality.