Originally posted by davey23uk
and the sys_context
If you mean the provided namspace USERENV, then I'm affraid there is no such parameter in there that would return SID or serial# for the session. Note that parameter SESSIONID in that namespace returns auditing session identifier, which is something totaly different from SID in V$SESSION and simmilar views.

As mentioned before, V$MYSTAT is the correct view to obtain your current SID.

SELECT sid FROM v$mystat WHERE ROWNUM = 1;