Add Between Symmetric as a postgres til

README.md

@@ -7,7 +7,7 @@ variety of languages and technologies. These are things that don't really
 warrant a full blog post. These are mostly things I learn by pairing with
 smart people at [Hashrocket](http://hashrocket.com/).
 
-_503 TILs and counting..._
+_504 TILs and counting..._
 
 ---
 
@@ -220,6 +220,7 @@ _503 TILs and counting..._
 - [Aggregate A Column Into An Array](postgres/aggregate-a-column-into-an-array.md)
 - [Assumed Radius Of The Earth](postgres/assumed-radius-of-the-earth.md)
 - [Auto Expanded Display](postgres/auto-expanded-display.md)
+- [Between Symmetric](postgres/between-symmetric.md)
 - [Change The Current Directory For psql](postgres/change-the-current-directory-for-psql.md)
 - [Checking Inequality](postgres/checking-inequality.md)
 - [Checking The Type Of A Value](postgres/checking-the-type-of-a-value.md)

postgres/between-symmetric.md

@@ -0,0 +1,41 @@
+# 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.