SQL query optimization practice - sql

Question

I want to pratice my skills in SQL query optimization for small databases.
I have created a very simple database:
I populated it with hunderds of thousands of entries and need some complex, stupid and unoptimized queries to make some tests and practice my optimization skills. Right now I tried to write some queries but they don't make much impact on the database.
Which key words should I use? Could someone help?

Answer 1

I find it hard to come up with queries that are really bad :-) Here are some ideas. Hopefully, others will come up with more. I suggest you put your tables (empty or even with some sample rows) in https://dbfiddle.uk/, where we can try our queries. I don't know for instance, if I don't have any syntax errors in below queries.
Find all videos that are either owned or commented by a user named xyz:
select distinct v.*
from videos v
left join user_have_videos uv on uv.video_id = v.video_id
left join users u1 on u1.user_id = uv.user_id
left join comments c on c.video_id = v.video_id
left join users u2 on u2.user_id = c.user_id
where u1.user_name = :user_name
or u2.user_name = :user_name;
Find all users that own at least three different videos:
select distinct u.*
from users u
left join user_have_videos uv1 on uv1.user_id = u.user_id
left join videos v1 on v1.video_id = uv1.video_id
left join user_have_videos uv2 on uv2.user_id = u.user_id
left join videos v2 on v2.video_id = uv2.video_id
left join user_have_videos uv3 on uv3.user_id = u.user_id
left join videos v3 on v3.video_id = uv3.video_id
where v1.video_id <> v2.video_id
and v1.video_id <> v3.video_id
and v2.video_id <> v1.video_id
and v2.video_id <> v3.video_id
and v3.video_id <> v1.video_id
and v3.video_id <> v2.video_id;
Find all users that commented on John Wayne:
select distinct u.*
from users u
join comments c on c.user_id = u.user_id
where lower(comments) like '%john wayne%';
And here is one where you shall find out what this does and make it better:
with cte(user_id, video_id, num) as
(
select u.user_id, v.video_id, 1
from videos v
join user_have_videos uv on uv.video_id = v.video_id
join users u on u.user_id = uv.user_id
union all
select u.user_id, min(v.video_id), min(cte.num) + 1
from videos v
join user_have_videos uv on uv.video_id = v.video_id
join users u on u.user_id = uv.user_id
join cte on cte.user_id = u.user_id
and cte.video_id < v.video_id
group by u.user_id
)
select distinct u.*
from users u
join cte on cte.user_id = u.user_id and cte.num >= 10;