Re: Ora 9.2 SQL: What's the fastest way to do this set operation...

You'll need to know that train T_A goes from point A to point C. So your table will contain TRAIN_NAME (T_A), SOURCE point (A) and DEST point (C). Similarly, you'll need to know that train T_B goes from point C to point B. So your table will contain SOURCE point (C) and DEST point (B). Now a simple SQL statment will find the two trains:

SELECT L1.TRAIN_NAME AS LEG1_TRAIN, L2.TRAIN_NAME AS LEG2_TRAIN FROM TRAIN_SCHEDULES L1, TRAIN_SCHEDULES L2 WHERE L1.DEST = L2.SOURCE
   AND L1.SOURCE = 'A' AND L2.DEST = 'B'; The above will give you any trains that start at point A for leg 1, arrive at point B for leg 2, and share a common midpoint for the overall leg.

The query above does not take into account that the LEG1_TRAIN may arrive after the departure of the LEG2_TRAIN. So you might have to modify the query to take that into account....

Cheers,
Brian

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Brian Peasland
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