Question

I want to fetch first 10 question haven’t seen by a user.
In order to do that, I need to store questions that have seen by that user into a table, named user_questions

So, my query is now:


SELECT * from questions WHERE questions.id NOT IN (select question_id from user_questions where user_id=1) LIMIT 0, 10


From what I have learnt, this query should be slower compare to similar join query. How could I construct a join query that produced the same result?

Answer 1

The equivalent join query is:

select q.*
from questions q left outer join
     user_questions uq
     on uq.question_id = q.id and
        uq.user_id = 1
where uq.user_id is null
limit 0, 30

One caution, though, the results will not be in random order. Typically with a limit clause, you would also specify an order by.