use Perl

Someone posted an SQL question in a forum I occasionally frequent. The setup:

CREATE TABLE tbl (num INTEGER);
INSERT INTO tbl VALUES (  1);
INSERT INTO tbl VALUES (  2);
INSERT INTO tbl VALUES (  3);
INSERT INTO tbl VALUES (  4);
INSERT INTO tbl VALUES (  9);
INSERT INTO tbl VALUES ( 10);
INSERT INTO tbl VALUES ( 11);
INSERT INTO tbl VALUES ( 20);
INSERT INTO tbl VALUES ( 21);
INSERT INTO tbl VALUES ( 22);
INSERT INTO tbl VALUES ( 23);
INSERT INTO tbl VALUES (100);
INSERT INTO tbl VALUES (101);
INSERT INTO tbl VALUES (102);
INSERT INTO tbl VALUES (103);

His question: is there a way to produce the following output?

start       end
----------  ----------
1           4
9           11
20          23
100         103

I didn’t think it was possible at first, but after some thinking and tinkering, I managed to come up with a solution:

SELECT s.num AS start, MIN( e.num ) AS end
FROM (
    SELECT num
    FROM tbl t1
    WHERE NOT EXISTS ( SELECT NULL FROM tbl t2 WHERE t2.num = t1.num - 1 )
    ORDER BY num
) s
CROSS JOIN (
    SELECT num
    FROM tbl t1
    WHERE NOT EXISTS ( SELECT NULL FROM tbl t2 WHERE t2.num = t1.num + 1 )
    ORDER BY num
) e
WHERE s.num <= e.num
GROUP BY s.num;