Recursion, the SQL way

Recursion reached SQL late, as the recursive common table expression. WITH RECURSIVE computes a fixed point, exactly the metro reachability from Part 2:

WITH RECURSIVE reach(src, dst) AS (
  SELECT src, dst FROM line
  UNION
  SELECT line.src, reach.dst
  FROM line JOIN reach ON line.dst = reach.src
)
SELECT * FROM reach;

But standard SQL allows only linear recursion: the recursive case may name reach just once. The doubly-recursive transitive closure and the parser cannot be expressed in plain SQL.