Oracle – the maximum length of a table name in Oracle

Aggregate the column by COUNT, then use a HAVING clause to find values that appear greater than one time.

SELECT column_name, COUNT(column_name)
FROM table_name
GROUP BY column_name
HAVING COUNT(column_name) > 1;

It's a sort of dummy table with a single record used for selecting when you're not actually interested in the data, but instead want the results of some system function in a select statement:

e.g. select sysdate from dual;

See http://www.adp-gmbh.ch/ora/misc/dual.html