SQL Koan

It's not that profound, but I sure do like it: a simple, elegant example of a self-join that gives a truth table for the NOT DISTINCT FROM operator.

WITH x(v) AS (VALUES (1),(2),(NULL))
  SELECT l.v, r.v, l.v = r.v AS equality,
    l.v IS NOT DISTINCT FROM r.v AS isnotdistinctfrom
  FROM x l, x r;

Thanks to Sam Mason via the PostgreSQL - general mailing list.