| Oracle® Database SQL Reference 10g Release 1 (10.1) Part Number B10759-01  | 
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USER returns the name of the session user (the user who logged on) with the datatype VARCHAR2. Oracle Database compares values of this function with blank-padded comparison semantics.
In a distributed SQL statement, the UID and USER functions together identify the user on your local database. You cannot use these functions in the condition of a CHECK constraint.
The following example returns the current user and the user's UID:
SELECT USER, UID FROM DUAL;