Union All Rows Including Duplicates

Two tables or sets of results can be joined together into a single result set using the union operator. When combining results with union, all duplicate rows will be removed from its result.

> select generate_series(1,4)
  union
  select generate_series(3,6)
  order by 1 asc;

 generate_series
-----------------
               1
               2
               3
               4
               5
               6
(6 rows)

Notice that despite both sides of the union having their own 3 and 4, those values each only show up once in the result.

If we don't want duplicates to be excluded, we can use union all.

> select generate_series(1,4)
  union all
  select generate_series(3,6)
  order by 1 asc;

 generate_series
-----------------
               1
               2
               3
               3
               4
               4
               5
               6
(8 rows)

In this case we have 8 rows instead of 6 with the values 3 and 4 each appearing twice.

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