Question

I have a table Transport that has 2 fks pointing a table Spot. those fks store origin and destination of the transport.

I need to do a query that gets the minimum distance between a point and origin and between the same point and destination.

This is what I have done, though this gets me 34 results and it should be 17. I know I’m doing it wrong, but I can’t get the query right:

SELECT LEAST(
   ST_Distance(ST_GeographyFromText('SRID=4326; POINT(-3 40)'), s.point), 
   ST_Distance(ST_GeographyFromText('SRID=4326; POINT(-3 40)'), s.point)
) FROM spot s RIGHT OUTER JOIN transport t 
ON t.origin = s.id OR t.destination = s.id;

Obviously I shouldn’t be using an OR here. I have tried to do double JOINs, but didn’t get it right.

Thanks for your help

Answer 1

You have to use two join, because your query need a table Spot meaning Origin, and a table Spot meaning Destination.

Try like this :

SELECT LEAST(
   ST_Distance(ST_GeographyFromText('SRID=4326; POINT(-3 40)'), sOrigin.point), 
   ST_Distance(ST_GeographyFromText('SRID=4326; POINT(-3 40)'), sDestination.point)
) FROM transport t
LEFT OUTER JOIN spot sOrigin ON t.origin = sOrigin.id 
LEFT OUTER JOIN spot sDestination ON t.destination = sDestination.id;