Better sql query for performace - sql

Question

I have a query that looks like :
select *
from Franvaro f
inner join Anvandare a on a.id = f.anvandare_id
where (
f.friskskriven = 'Y'
and not exists (
select *
from sscpost
where franvaro_id = f.id
and friskskrivenpost = 'Y'
)
)
or (
f.startdatum <= '2020-04-05'
and not exists (
select *
from sscpost
where franvaro_id = f.id
and friskskrivenpost = 'Y'
)
)
and a.anstallningtyp = 'A'
;
I wonder if this query could be written in another way with the same output/result in order to have a better performance or better structure.I have tested also with this query(see below), but the result is not the same compared to the other query.
select *
from Franvaro f
inner join Anvandare a on a.id = f.anvandare_id
where f.friskskriven = 'Y'
or f.startdatum <= '2020-04-05'
and not exists (
select *
from sscpost
where franvaro_id = f.id
and friskskrivenpost = 'Y'
)
and a.anstallningtyp = 'A'
;

Answer 1

This might not matter with modern optimizers anymore, but in your EXISTS clause, you don't actually need anything returned. Therefore you might return 1 or NULL or something. I'm showing my age.
At the end of the day, the best way to optimize queries is to learn how to read their EXPLAIN PLAN's. Every SQL system works differently, and there sometimes isn't 1 best way. It will also depend on the size of these tables and what the data looks like.
But there is nothing inherently wrong with your code, it looks fine.
select *
from Franvaro f
inner join Anvandare a on a.id = f.anvandare_id
where (
f.friskskriven = 'Y'
and not exists (
select 1
from sscpost
where franvaro_id = f.id
and friskskrivenpost = 'Y'
)
)
or (
f.startdatum <= '2020-04-05'
and not exists (
select 1
from sscpost
where franvaro_id = f.id
and friskskrivenpost = 'Y'
)
)
and a.anstallningtyp = 'A'
;

Answer 2

Try:
SELECT *
FROM Franvaro f
INNER JOIN Anvandare a ON a.id = f.anvandare_id
WHERE a.anstallningtyp = 'A'
AND ( f.friskskriven = 'Y' OR f.startdatum <= '2020-04-05' )
AND NOT EXISTS
(
SELECT null
FROM sscpost
WHERE franvaro_id = f.id
AND friskskrivenpost = 'Y'
)
Mainly we combine the NOT EXISTS statement into one call