Add Union All Rows Including Duplicates as a Postgres til

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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 - [Turning Timing On](postgres/turn-timing-on.md)
 - [Two Ways To Compute Factorial](postgres/two-ways-to-compute-factorial.md)
 - [Types By Category](postgres/types-by-category.md)
+- [Union All Rows Including Duplicates](postgres/union-all-rows-including-duplicates.md)
 - [Use A psqlrc File For Common Settings](postgres/use-a-psqlrc-file-for-common-settings.md)
 - [Use Argument Indexes](postgres/use-argument-indexes.md)
 - [Use Not Valid To Immediately Enforce A Constraint](postgres/use-not-valid-to-immediately-enforce-a-constraint.md)

postgres/union-all-rows-including-duplicates.md

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+# Union All Rows Including Duplicates
+
+Two tables or sets of results can be joined together into a single result set
+using [the `union`
+operator](https://www.postgresql.org/docs/current/queries-union.html). When
+combining results with `union`, all duplicate rows will be removed from its
+result.
+
+```sql
+> 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`.
+
+```sql
+> 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.
+
+[source](https://www.postgresqltutorial.com/postgresql-union/)