mysql - 通过 MySQL 查询优化组

Question

我有一个 orders 表，其中连续保存一个用户订单的一个项目。因此每个用户可以出现在许多行中。每个订单行都有一个状态 - PENDING、APPROVED 等。我想计算每个用户每个状态的订单数量，即有多少订单已批准，有多少订单仍在等待处理等等。我提出了以下查询:

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

问题是它太慢了!我的 orders 表中有大约 5000 条记录，执行需要几分钟。请指教如何优化。

Answer 1

您可以使用条件聚合来消除子查询:

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