How to join with unnest function in athena?

Question

i have this query on Athena

trip.id as tripid
, segment.id as segmentid
, segment.distance as mileage
, segment.maxspeed as maxspeed
, segment.duration as duration
, segment.roadtype as roadtype
, segment.timeslotdata as timeslots
, extract( week from (from_unixtime(trip.referencedate /1000))) as weekyear
, extract( year from (from_unixtime(trip.referencedate /1000))) as year
, extract( month from (from_unixtime(trip.referencedate/1000))) as month
, unn.firstpositionlat
, unn.firstpositionlong
from
trip
, UNNEST(segments) as t(segment)
left join
(
select
position.latitude as firstpositionlat
, position.longitude as firstpositionlong
, position.id as id
from
trip
, UNNEST(segments) as t(segment)
, UNNEST(segment.positions) as t(position)
where
anno = 2021
and mese = 01
and giorno = 01
and tscid = 'XXX'
)
unn
on
segment.startpositionid = unn.id
where
anno = 2021
and mese = 01
and giorno = 01
and tscid = 'XXX'

the problem is that i can't join because of an error on line 16:19, and i have this error

SYNTAX_ERROR: line 16:19: Column 'segments' cannot be resolved

i can't figure out where the problem is. Without the ON statement, this works well.

thank you in advance

Answer 1

Presto (underlying SQL engine for Athena) supports UNNEST only on the right side of CROSS JOIN . For example:

-- sample data
WITH dataset (id, animals) AS (
    values (1, ARRAY['dog', 'cat', 'bird']),
    (2, ARRAY['cow', 'pig'])
) 

-- query
SELECT id, animals, a
FROM dataset
CROSS JOIN UNNEST(animals) AS t (a);

Output:

id	animals	a
1	[dog, cat, bird]	dog
1	[dog, cat, bird]	cat
1	[dog, cat, bird]	bird
2	[cow, pig]	cow
2	[cow, pig]	pig

It seems that shorthand notation ( from trip, UNNEST(segments) as t(segment) ) is not working correctly when followed by another join with on condition, try to just expand it to the full form:

from trip
cross join UNNEST(segments) as t(segment)
left join (...)
on ...