There are exhaustive notes on how the
Oracle SQL UNION operator is
used to merge separate SQL queries, but there is an important
difference between UNION and UNION ALL.
The Oracle SQL UNION differs from the Oracle UNION ALL primarily
because it does not filter out duplicitous rows.
The UNION SQL operator returns only the unique rows that appear in
either result, while the UNION ALL operator returns all rows in both
queries, including duplicate rows.
See how Oracle 11gR2
improves the execution speed of UNION ALL queries.
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