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The Oracle DUAL table
dual is a table which is created by oracle along with the data dictionary. It consists of exactly one column whose name is dummy and one record. The value of that record is X.
As dual contains exactly one row (unless someone fiddled with it), it is guaranteed to return exactly one row in select statements. Therefor, dual is the prefered table to select a pseudo column (such as sysdate
select sysdate from dual
Although it is possible to delete the one record, or insert additional records, one really should not do that!.