SQL how to SELECT an id based on multiple rows conditions - sql

Question

I am working with nodejs and postgresql. My postgresql relation has 3 columns:
id
lesson_id
tag_id.
A lesson could belong to 1 or multiple tags.
I am trying to select all the lesson whose belongs to the requested tags.
For example
tags requested are id 10 and 2, the query should response with lesson id = 3
tags requested are id 1 and 17, the query should response with lesson id = 6
tag requested is 3, the query should response with lessons id 1, 2, 4
I have tried some sql queries like this one:
const selectLessonByTag = await pgClient.query(
`SELECT DISTINCT ON (lesson_id)
lesson_id FROM "lesson_has_tag"
WHERE tag_id = $1 AND tag_id = $2
GROUP BY lesson_id
ORDER BY lesson_id`,
[2,10]);
but it's not the expected answer.

Answer 1

You can use not exists like so:
select distinct lesson_id
from lesson_tags as lt1
where not exists (
select *
from (values (10), (2)) as required_tags(tag_id)
where not exists (
select *
from lesson_tags as lt2
where lt2.lesson_id = lt1.lesson_id and lt2.tag_id = required_tags.tag_id
)
)
It is difficult to digest so little explanation:
There is a table valued constructor called required_tags containing values 10 and 2
The inner query tests if 10 or 2 do not exist for a lesson from the outer query
If the inner query does not produce a match the outer row selected
DB<>Fiddle