Add Between Symmetric as a postgres til

postgres/between-symmetric.md

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+# Between Symmetric
+
+PostgreSQL's `between` construct allows you to make a comparison _between_
+two values (numbers, timestamps, etc.).
+
+```sql
+> select * from generate_series(1,10) as numbers(a) where numbers.a between 3 and 6;
+ a
+---
+ 3
+ 4
+ 5
+ 6
+```
+
+If you supply an empty range by using the larger of the two values first, an
+empty set will result.
+
+```sql
+> select * from generate_series(1,10) as numbers(a) where numbers.a between 6 and 3;
+ a
+---
+```
+
+Tacking `symmetric` onto the `between` construct is one way to avoid this
+issue.
+
+```sql
+> select * from generate_series(1,10) as numbers(a) where numbers.a between symmetric 6 and 3;
+ a
+---
+ 3
+ 4
+ 5
+ 6
+```
+
+> BETWEEN SYMMETRIC is the same as BETWEEN except there is no requirement
+> that the argument to the left of AND be less than or equal to the argument
+> on the right. If it is not, those two arguments are automatically swapped,
+> so that a nonempty range is always implied.