Is this type of query possible (select with rows stopping based on match)

Question

I'm trying to write a SQL query and suspect it can't be done. Or at least, I can't mentally grasp it, despite various stabs with subqueries. I've simplified it by changing the metaphor here a little.
Let's say I have a truck in some city (say, Chicago). There are various roads leading out of that city, with routes we'll label according to compass points (N, NE, SW, etc.) Each truck can carry only one shipment. It starts going along its road, and as soon as it comes to a city that needs its shipment, it stops.
Yes, a cooked example, but it's similar to my problem.
Here is a table:
TABLE: truck_routes
scenario (integer - different possible scenarios)
route (NE, SW, etc. - 8 compass directions)
destination (cities along that road)
sequence_number (order in which to go to those cities)
So if I have a trucks in Chicago, I can say
SELECT route, sequence_number, destination
FROM truck_routes
WHERE scenario = 10 (or whatever)
ORDER BY road_out, sequence_number;
And I'd get a report like this:
SE 1 Gary
SE 2 Valparaiso
SE 3 Plymouth
SE 4 Warsaw
NE 1 Michigan City
NE 2 Kalamazoo
NE 3 Grand Rapids
NE 4 Cadillac
NE 5 Traverse City
So far so good. I have another table with a list of cities that need to receive the shipment:
TABLE: cities
scenario (integer - different possible scenarios)
route (NE, SW, etc. - 8 compass directions)
destination (same keys as in truck_routes)
e.g.:
10 SE Valparaiso
10 SE Warsaw
10 NE Cadillac
10 NE Traverse City
Now...the tricky part...how do I rewrite the query above so that after a truck hits a city that needs a shipment (in that scenario) it stops? In other words, I want a report (for all routes) of what cities the truck will stop at - and as mentioned, it stops at the first one it makes a delivery. I don't want to see rows beyond that. So the report would look like this:
SE 1 Gary
SE 2 Valparaiso
NE 1 Michigan City
NE 2 Kalamazoo
NE 3 Grand Rapids
NE 4 Cadillac
Gary shows up because it doesn't require a delivery. Valparaiso (SE route, sequence_number 2) shows up because it takes the delivery. The rest of the Southeast route doesn't show up. Same thing for the NE route.
There are scenarios where I'd actually want to stop at the city just prior to the delivery destination, etc., and of course in the real world it's more complicated, but I think if I could get this model working I could flesh it out into a real-world query.
I can certainly code something in PL/SQL to do this, or do it in application logic - but I'm wondering if there is a way to do it in a single SQL statement? I suspect there's something that could be built with subqueries and sorting, but...I can't quite reach it ;-)

Answer 1

WITH report AS(
    SELECT 'SE' route, '1' seq_num, 'Gary' dest  FROM DUAL UNION ALL
    SELECT 'SE' route, '2' seq_num, 'Valparaiso' dest FROM DUAL UNION ALL
    SELECT 'SE' route, '3' seq_num, 'Plymouth' dest FROM DUAL UNION ALL
    SELECT 'SE' route, '4' seq_num, 'Warsaw' dest FROM DUAL UNION ALL
    SELECT 'NE' route, '1' seq_num, 'Michigan City' dest FROM DUAL UNION ALL
    SELECT 'NE' route, '2' seq_num, 'Kalamazoo' dest FROM DUAL UNION ALL
    SELECT 'NE' route, '3' seq_num, 'Grand Rapids' dest FROM DUAL UNION ALL
    SELECT 'NE' route, '4' seq_num, 'Cadillac' dest FROM DUAL UNION ALL
    SELECT 'NE' route, '5' seq_num, 'Traverse City' dest FROM DUAL 
cities AS (
    SELECT '10' scenario, 'SE' route, 'Valparaiso' city FROM DUAL UNION ALL
    SELECT '10' scenario, 'SE' route, 'Warsaw' city FROM DUAL UNION ALL
    SELECT '10' scenario, 'NE' route, 'Cadillac' city FROM DUAL UNION ALL
    SELECT '10' scenario, 'NE' route, 'Traverse City' city FROM DUAL
need_a_shipment AS (
      SELECT r.*
          , CASE WHEN EXISTS( SELECT 'Y' FROM cities c WHERE c.route = r.route AND c.city = r.dest )
                 THEN 'Y'
                 ELSE 'N'
            END need_a_shipment
    FROM report r
SELECT  route
      , seq_num
      , dest
      , need_a_shipment
FROM need_a_shipment n
WHERE seq_num <= (  SELECT MIN(seq_num) FROM need_a_shipment
                    WHERE route = n.route AND need_a_shipment = 'Y' )
ORDER BY route DESC, seq_num
ROUTE SEQ_NUM DEST          NEED_A_SHIPMENT
SE    1       Gary          N              
SE    2       Valparaiso    Y              
NE    1       Michigan City N              
NE    2       Kalamazoo     N              
NE    3       Grand Rapids  N              
NE    4       Cadillac      Y