For 11g, ORACLE_SID is used to distinguish this instance from other Oracle Database instances that you may create later and run concurrently on the same host computer. The maximum number of characters for ORACLE_SID is 12, and only letters and numeric digits are permitted. On some platforms, the SID is case-sensitive. http://download.oracle.com/docs/cd/B28359_01/server.111/b28310/create003.htm#ADMIN11074
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