Turn a Mysql Subquery in a Join - mysql

Question

How can I turn this subquery in a JOIN?
I read that subqueries are slower than JOINs.
SELECT
reklamation.id,
reklamation.titel,
(
SELECT reklamation_status.status
FROM reklamation_status
WHERE reklamation_status.id_reklamation = reklamation.id
ORDER BY reklamation_status.id DESC
LIMIT 1
) as status
FROM reklamation
WHERE reklamation.aktiv=1

Answer 1

This should do it:
SELECT r.id, r.titel, MAX(s.id) as status
FROM reklamation r
LEFT JOIN reklamation_status s ON s.id_reklamation = r.id
WHERE r.aktiv = 1
GROUP BY r.id, r.titel
The key point here is to use aggregation to manage the cardinality between reklamation and reklamation_status. In your original code, the inline subquery uses ORDER BY reklamation_status.id DESC LIMIT 1 to return the highest id in reklamation_status that corresponds to the current reklamation. Without aggregation, we would probably get multiple records in the resultset for each reklamation (one for each corresponding reklamation_status).
Another thing is to consider is the type of the JOIN. INNER JOIN would filter out records of reklamations that do not have a reklamation_status: the original query with the inline subquery does not behave like that, so I chose LEFT JOIN. If you can guarantee that every reklamation has at least one child in reklamation_status, you can safely switch back to INNER JOIN (which might perform more efficiently).
PS:
I read that subqueries are slower than JOINs.
This is not a universal truth. It depends on many factors and cannot be told without seeing your exact use case.

Answer 2

Using JOIN query can be rewritten to:
SELECT reklamation.id, reklamation.titel, reklamation_status.status
FROM reklamation
JOIN reklamation_status ON reklamation_status.id_reklamation = reklamation.id
WHERE reklamation.aktiv=1

Answer 3

What you read is incorrect. Subqueries can be slower, faster, or the same as joins. I would write the query as:
SELECT r.id, r.titel,
(SELECT rs.status
FROM reklamation_status rs
WHERE rs.id_reklamation = r.id
ORDER BY rs.id DESC
LIMIT 1
) as status
FROM reklamation r
WHERE r.aktiv = 1;
For performance, you want an index on reklamation_status(id_reklamation, id, status).
By the way, this is a case where the subquery is probably the fastest method for expressing this query. If you attempt a JOIN, then you need some sort of aggregation to get the most recent status. That can be expensive.