"union" and "union all"

[CeciMego]

can anyone tell me what's the difference between these?
i can tell "union all" is a bit faster... but what is the real difference?

[oratune]

UNION sorts the data from the combined result sets and removes any duplicates between them, ensuring that only distinct records are returned.

UNION ALL does no such 'cleanup', and, therefore, is much faster, since it simply UNION's all data records and returns the combined result.

[marist89]

UNION will return the unique rows, UNION ALL will return all the rows. For example:

SQL> select * from a;

A
----------
A
B
C

SQL> select * from b
  2  ;

B
----------
B
C
D

SQL> select a from a
  2  union 
  3  select b from b;

A
----------
A
B
C
D

SQL> select a from a
  2  union all
  3  select b from b;

A
----------
A
B
C
B
C
D

6 rows selected.