PostgreSQL: create table with unique timestamp for all rows - sql

Question

I have a record of users' trips with begin/end positions and time in a table like this:
CREATE TABLE trips(id integer, start_timestamp timestamp with time zone,
session_id integer, start_lat double precision,
start_lon double precision, end_lat double precision,
end_lon double precision, mode integer);
INSERT INTO trips (id, start_timestamp, session_id, start_lat,start_lon,end_lat,end_lon,mode)
VALUES (563097015,'2017-05-20 17:47:12+01', 128618, 41.1783308,-8.5949878, 41.1784478, -8.5948463, 0),
(563097013, '2017-05-20 17:45:29+01', 128618, 41.1781344, -8.5951169, 41.1782919, -8.5950689, 0),
(563097011, '2017-05-20 17:43:41+01', 128618, 41.1781196, -8.5954075, 41.1782139, -8.5950689, 0),
(563097009, '2017-05-20 17:41:48+01', 128618, 41.1782497, -8.595197, 41.1781101, -8.5954124, 0),
(563097003, '2017-05-20 17:10:29+01', 128618, 41.1832512, -8.6081606, 41.1782561, -8.5950259, 0)
And in the second table is the records of raw gps traces for all the trips similar to:
CREATE TABLE gps_traces (session_id integer, seconds integer, lat double precision,
lon double precision, speed double precision);
INSERT INTO gps_traces (session_id, seconds , lat , lon , speed )
VALUES (128618,1495296443,41.1844471,-8.6065158,1.35148),
(128618,1495296444,41.1844482,-8.6065303,1.28004),
(128618,1495296445,41.1844572,-8.6065503,1.46086),
(128618,1495296446,41.1844541,-8.6065691,1.23),
(128618,1495296446,41.1844589,-8.6065861, 1.22919),
(128618,1495296447,41.1844587, -8.6066043, 1.30188),
(128618, 1495296448, 41.1844604, -8.6066261, 1.43126),
(128618, 1495296449, 41.184471, -8.6066412, 1.55003),
(128618,1495296450, 41.1844715, -8.6066572, 1.29062),
(128618,1495296450, 41.1844707, -8.6066736, 1.3618)
From this I want to create a new table mytable containing GPS joining these tables on session_id, like so:
CREATE TABLE mytable AS SELECT id, seconds, lat, lon, speed, mode
FROM trips t
JOIN gps_traces g
ON t.session_id=g.session_id
However, in the new table, I want to ensure that for rows recorded twice at same unix timestamp in a trip, only only is selected into my new table. For example in this case:
SELECT * FROM mytable WHERE id = 563097003;
+-----------+------------+------------+------------+---------+------+
| id | seconds | lat | lon | speed | mode |
+-----------+------------+------------+------------+---------+------+
| 563097003 | 1495296443 | 41.1844471 | -8.6065158 | 1.35148 | 0 |
| 563097003 | 1495296444 | 41.1844482 | -8.6065303 | 1.28004 | 0 |
| 563097003 | 1495296445 | 41.1844572 | -8.6065503 | 1.46086 | 0 |
| 563097003 | 1495296446 | 41.1844541 | -8.6065691 | 1.23 | 0 |
| 563097003 | 1495296446 | 41.1844589 | -8.6065861 | 1.22919 | 0 |
| 563097003 | 1495296447 | 41.1844587 | -8.6066043 | 1.30188 | 0 |
| 563097003 | 1495296448 | 41.1844604 | -8.6066261 | 1.43126 | 0 |
| 563097003 | 1495296449 | 41.184471 | -8.6066412 | 1.55003 | 0 |
| 563097003 | 1495296450 | 41.1844715 | -8.6066572 | 1.29062 | 0 |
| 563097003 | 1495296450 | 41.1844707 | -8.6066736 | 1.3618 | 0 |
| 10 rows | | | | | |
+-----------+------------+------------+------------+---------+------+
Column seconds is the Unix timestamp. As shown, we can see rows having more than 1 unique timestamp count at 1495296446 and 1495296450. I would like to ensure that for each trip, records are selected into the new table with unique timestamp (so in the case above, only one recorded should selected into the new table). I illustrate that in this db<>fiddle.
EDIT
Expected output:
+-----------+------------+------------+------------+---------+------+
| id | seconds | lat | lon | speed | mode |
+-----------+------------+------------+------------+---------+------+
| 563097003 | 1495296443 | 41.1844471 | -8.6065158 | 1.35148 | 0 |
| 563097003 | 1495296444 | 41.1844482 | -8.6065303 | 1.28004 | 0 |
| 563097003 | 1495296445 | 41.1844572 | -8.6065503 | 1.46086 | 0 |
| 563097003 | 1495296446 | 41.1844541 | -8.6065691 | 1.23 | 0 |
| 563097003 | 1495296447 | 41.1844587 | -8.6066043 | 1.30188 | 0 |
| 563097003 | 1495296448 | 41.1844604 | -8.6066261 | 1.43126 | 0 |
| 563097003 | 1495296449 | 41.184471 | -8.6066412 | 1.55003 | 0 |
| 563097003 | 1495296450 | 41.1844715 | -8.6066572 | 1.29062 | 0 |
| 8 rows | | | | | |
+-----------+------------+------------+------------+---------+------+

Answer 1

Use DISTINCT ON:
CREATE TABLE mytable AS
SELECT DISTINCT ON (t.session_id, seconds) id, seconds, lat, lon, speed, mode
FROM trips t JOIN
gps_traces g
ON t.session_id = g.session_id
ORDER BY t.session_id, seconds;
Note: I would expect you to include session_id in the new table as well.
Thanks to #Abelisto, it turns out that the following modification to this answer works as intended.
CREATE TABLE mytable AS SELECT DISTINCT ON (id, seconds)id,
seconds, lat, lon, speed, mode
FROM trips t
JOIN gps_traces g
ON t.session_id=g.session_id
ORDER BY id, seconds
Here is a db<>fiddle.