Between Symmetric

PostgreSQL's between construct allows you to make a comparison between two values (numbers, timestamps, etc.).

> 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.

> 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.

> 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.