How to select from relationship based on the height value of field postgres? - postgresql

Question

i have three tables vehicles and trips and componentValues they are related to each other by
vehicles -> trips -> componentValues
vehicles Table : id, ...
trips Table: id, vehicle_id, ...
componentValues Table: id, trip_id, damage, ...
and i'm trying to get all the trips with the highest damage component form the componentValues table like this
SELECT *
FROM (select * from trips WHERE "trips"."vehicle_id" = '7') as t
LEFT JOIN (
select * from "component_values"
where trip_id = '85'
order by damage
desc nulls last limit 1
) as h on t.id = h.trip_id
how can i change the line where trip_id = '85' to be dynamic or is their another way to do this and many thanks in advance.
expected result:
UPDATE
i have did some query that get what i want but how can i improve it by not using sub queries in the select statement
select * ,
(select damage from "component_values" where trip_id = trips.id order by damage desc nulls last limit 1) as h_damage,
(select damage from "component_values" where trip_id = trips.id order by damage asc nulls first limit 1) as l_damage,
(select component_types.name from "component_values" left join component_types on component_values.component_type_id = component_types.id where trip_id = trips.id order by damage desc nulls last limit 1) as hc_damage,
(select component_types.name from "component_values" left join component_types on component_values.component_type_id = component_types.id where trip_id = trips.id order by damage asc nulls first limit 1) as lc_damage
from trips
WHERE trips."vehicle_id" = '7'

Answer 1

I think you want distinct on:
select distinct on (t.id) t.*, dv.damage
from trips t join
component_values cv
on cv.trip_id = t.id
where t.vehicle_id = 7 -- not sure if this is needed
order by t.id, cv.damage desc nulls last;
distinct on is usually the most efficient method in Postgres. You can also do this with window functions:
select distinct on (t.id) t.*, cv.damage
from trips t join
(select cv.*,
row_number() over (partition by cv.trip_id, cv.damage desc nulls last) as seqnum
from component_values cv
) cv
on cv.trip_id = t.id and cv.seqnum = 1
where t.vehicle_id = 7; -- not sure if this is needed

Answer 2

I think you want a lateral join.
SELECT *
FROM (select * from trips WHERE "trips"."vehicle_id" = '7') as t
LEFT JOIN lateral (
select * from "component_values"
where trip_id = t.id
order by damage
desc nulls last limit 1
) as h on true
Although I don't think there is a reason for the first subquery, so:
SELECT *
FROM trips
LEFT JOIN lateral (
select * from "component_values"
where trip_id = trips.id
order by damage
desc nulls last limit 1
) as h on true
WHERE "trips"."vehicle_id" = '7'