Joining two point datasets using distance as the relation between them.
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September 14, 2016 09:14
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SQL: JOIN by proximity
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WITH | |
-- some cities, just for this example | |
cities AS ( | |
SELECT | |
cartodb_id,name, | |
the_geom, | |
st_x(the_geom) AS lon, | |
st_y(the_geom) AS lat | |
FROM populated_places | |
WHERE megacity = 1 | |
), | |
-- same cities slightly moved | |
moved_cities AS ( | |
SELECT | |
cartodb_id, | |
concat(name, ' 2') AS other_name , | |
CDB_LatLng( | |
lat*(1+random()*0.0001), -- randomizing a bit the coordinates | |
lon*(1+random()*0.0001) | |
) AS the_geom | |
FROM cities | |
), | |
-- get the id of the closest geometry | |
with_ids AS ( | |
SELECT | |
c.*, | |
( | |
-- the trick is here, | |
-- returning the id of the closest geometry | |
-- important to use the <#> operator so | |
-- you work with the index, way faster! | |
SELECT cartodb_id AS moved_id | |
FROM moved_cities m | |
ORDER BY c.the_geom <#> m.the_geom | |
LIMIT 1 | |
) | |
FROM cities c | |
) | |
-- JOIN with the moved query | |
SELECT | |
c.cartodb_id, | |
c.name, | |
m.other_name | |
FROM with_ids c | |
JOIN moved_cities m | |
ON c.moved_id = m.cartodb_id |
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