SQL查询-在计数大于值的地方选择max

Question

I've got two tables with the following structure:

id int, 
question text, 
answer text, 
level int

qid int, 
attempts int, 
completed boolean (qid means question id)

Now my questions is how to construct a query that selects the max level where the count of correct questions is greater than let's say 30.

I created this query, but it doesn't work and I don't know why.

SELECT MAX(Questions.level) 
FROM Questions, Progress 
WHERE Questions.id = Progress.qid AND Progress.completed = 1 
GROUP BY  Questions.id, Questions.level 
Having COUNT(*) >= 30

I would like to have it in one query as I suspect this is possible and probably the most 'optimized' way to query for it. Thanks for the help!

Answer 1

This sort of construct will work. You can figure out the details.

select max(something) maxvalue
from SomeTables
join (select id, count(*) records
from ATable
group by id) temp on ATable.id = temp.id
where records >= 30

Answer 2

Do it step by step rather than joining the two tables. In an inner select find the questions (ie the question ids) that were answered 30 times correctly. In an outer select find the corresponding levels and get the maximum value:

select max(level) 
from questions
where id in
(
  select qid
  from progress 
  where completed = 1
  group by qid
  having count(*) >= 30
);