How to determine database compatibility level

COMPATIBILITY allows you to use a new release of Oracle, while at the same time guaranteeing backward compatibility with an earlier release.

In order to determine your current database compatibility level you can query from view database_compatible_level.

SQL> col value for a11
SQL> col description for a50
SQL> select * from database_compatible_level;

VALUE DESCRIPTION
----------- --------------------------------------------------
11.1.0.0.0 Database will be completely compatible with this s
oftware version

Alternatively you can check your initialization parameter COMPATIBLE setting.
On Sql*plus simply issue,

SQL> show parameter compatible

NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
compatible string 11.1.0.0.0

You can also query from v$parameter,
SQL> select value from v$parameter where name='compatible';

VALUE
-----------
11.1.0.0.0

SQL> select value from v$spparameter where name='compatible';

VALUE
-----------
11.1.0.0.0