Optimizing a group by MySQL query

Question

I have an orders table that holds one item for one user order in a row. So each user can appear on many rows. Each order line has a status - PENDING, APPROVED and so on. I want to count for each user the number of orders per status, that is how many orders are approved, how many are still pending and so on. I came up with the following query:

SELECT 
    u.email, u.id,
    (SELECT count(status) FROM orders WHERE user_id = o.user_id AND status = 'PENDING') pending,
    (SELECT count(status) FROM orders WHERE user_id = o.user_id AND status = 'APPROVED') approved,
    (SELECT count(status) FROM orders WHERE user_id = o.user_id AND status = 'REJECTED') rejected,
    (SELECT count(status) FROM orders WHERE user_id = o.user_id AND status = 'ERROR') error
FROM orders o INNER JOIN users u ON o.user_id = u.id
GROUP BY o.user_id, u.email

The problem is that it is mad slow! I have about 5 000 records in orders table and it makes minutes to execute. Please advice how to optimize it.

Answer 1

You can get rid of the sub-queries using conditional aggregation:

SELECT 
    u.email, u.id,
    COUNT(CASE WHEN status = 'PENDING' THEN 1 END) pending,
    COUNT(CASE WHEN status = 'APPROVED' THEN 1 END) approved,
    COUNT(CASE WHEN status = 'REJECTED' THEN 1 END) rejected,
    COUNT(CASE WHEN status = 'ERROR' THEN 1 END) error
FROM orders o INNER JOIN users u ON o.user_id = u.id
GROUP BY o.user_id, u.email