Using multiple recursive common table expressions

A recursive query may include multiple recursive queries, as long as they are disjoint. It may also include a mix of recursive and non-recursive common table expressions. The RECURSIVE keyword must be present if at least one of the common table expressions is recursive.

For example, the following query—which returns the same result as the previous query—uses a second, non-recursive common table expression to select the length of the shortest route. The definition of the second common table expression is separated from the definition of the first by a comma.

WITH RECURSIVE
  trip ( route, destination, previous, distance, segments ) AS
    ( SELECT CAST( origin || ', ' || destination AS VARCHAR(256) ),
             destination, origin, distance, 1
      FROM travel
      WHERE origin = 'Kitchener'
      UNION ALL
      SELECT route || ', ' || v.destination,
             v.destination,
             v.origin,
             t.distance + v.distance,
             segments + 1
      FROM trip t JOIN travel v ON t.destination = v.origin
      WHERE v.destination <> 'Kitchener'
        AND v.destination <> t.previous
        AND v.origin      <> 'Pembroke'
        AND segments
              < ( SELECT count(*)/2 FROM travel ) ),
  shortest ( distance ) AS                 -- Additional,
    ( SELECT MIN(distance)                 -- non-recursive
      FROM trip                            -- common table
      WHERE destination = 'Pembroke' )     -- expression
SELECT route, distance, segments FROM trip
WHERE destination = 'Pembroke' AND
      distance < 1.5 * ( SELECT distance FROM shortest )
ORDER BY distance, segments, route;

Like non-recursive common table expressions, recursive expressions, when used within stored procedures, may contain references to local variables or procedure parameters. For example, the best_routes procedure, defined below, identifies the shortest routes between the two named cities.

CREATE PROCEDURE best_routes (
   IN initial VARCHAR(10),
   IN final   VARCHAR(10)
)
BEGIN
  WITH RECURSIVE
      trip ( route, destination, previous, distance, segments ) AS
  ( SELECT CAST( origin || ', ' || destination AS VARCHAR(256) ),
           destination, origin, distance, 1
    FROM travel
    WHERE origin = initial
      UNION ALL
    SELECT route || ', ' || v.destination,
           v.destination,            -- current endpoint
           v.origin,                 -- previous endpoint
           t.distance + v.distance,  -- total distance
           segments + 1              -- total number of segments
    FROM trip t JOIN travel v ON t.destination = v.origin
    WHERE v.destination <> initial     -- Don't return to start
      AND v.destination <> t.previous  -- Prevent backtracking
      AND v.origin      <> final       -- Stop at the end
      AND segments                     -- TERMINATE RECURSION!
            < ( SELECT count(*)/2 FROM travel ) )
  SELECT route, distance, segments FROM trip
  WHERE destination = final AND
        distance < 1.4 * ( SELECT MIN( distance )
                           FROM trip
                           WHERE destination = final )
  ORDER BY distance, segments, route;
END;

The following statement demonstrates how to call the above procedure.

CALL best_routes ( 'Pembroke', 'Kitchener' );

Send feedback about this page via email or DocCommentXchange