postgreSQL - find next lesson for specific instruments - sql

Question

I'm trying to write a PostgreSQL query to list the two instruments that are available for rent, with the lowest monthly rental fee, also tell when the next lesson for each listed instrument is scheduled. I have these two tables:
//Table lesson
lesson_id | instrument_type | start
001 | 01 | 2021-02-01
002 | 01 | 2021-02-02
003 | 02 | 2021-02-04
004 | 02 | 2021-02-05
005 | 03 | 2021-02-06
//Table instrument
instrument_id | fee_per_month | availability
01 | 300 | yes
02 | 400 | no
03 | 500 | yes
And I want:
instrument_type | fee_per_month | lesson_id | start
01 | 300 | 001 | 2021-02-01
03 | 500 | 005 | 2021-02-06
SQL is new to me, and I have tried my best but didn't succeed:
SELECT
instrument.type AS "instrument",
instrument.fee_per_month AS "fee/month",
lesson.start AS "next lesson"
FROM instrument, lesson
LEFT JOIN LATERAL (
SELECT lesson.*
FROM lesson
WHERE lesson.start >= current_timestamp AND lesson.instrument_type = instrument.type
ORDER BY lesson.start
limit 1
) lesson on true
GROUP BY "instrument", "rent/month", "next lesson"
ORDER BY "rent/month"
limit 2;
How should I do it correctly?

Answer 1

Hmmm . . . I think you are sort of on the right track. If I understand correctly:
SELECT i.instrument_id AS "instrument", i.fee_per_month,
l.lesson_id, l.start AS "next lesson"
FROM instrument i LEFT JOIN LATERAL
(SELECT l.*
FROM lesson l
WHERE l.start >= current_timestamp AND
l.instrument_id = i.instrument_id
ORDER BY l.start
LIMIT 1
) l
ON true
WHERE i.availability = 'yes'
ORDER BY i.fee_per_month
LIMIT 2;
This fixes up the query, the logic, and the names of things. Here is a db<>fiddle.
The major differences are:
No Cartesian product between instrument and lesson.
Introducing table aliases so the query is easier to write and to read.
Remove the GROUP BY. No aggregation is necessary.