mysql - MySQL 查询中的子选择

Question

我想在查询中为 3 个变量添加别名:total_time_taken、average 和 request_count。

平均值旨在计算total_time_taken/request_count以返回平均值，但它给了我一个语法错误

You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'select(created_at, assigned_at, SUM(TIMESTAMPDIFF(SECOND, requests.created_at, r' at line 4

查询如下。

select
    *,
    COUNT(*) as request_count,
    select(created_at, assigned_at, SUM(TIMESTAMPDIFF(SECOND, requests.created_at, requests.assigned_at)) from requests) as total_time_taken,
    total_time_taken / request_count as average
from
    `requests`
where
    `deleted_at` is null
and
    `submitted_at` >= '2017-03-30 00:00:00'
and
    `requests`.`deleted_at` is null
group by
    `engineer_id`
limit 5

Answer 1

我不明白子查询的意义，为什么不直接取每个工程师聚合的总和呢？另外，您正在使用 GROUP BY 执行 SELECT *，这通常不起作用，而且通常是错误的，因为它会包含非聚合列。相反，只需选择 engineer_id 或其他列的聚合即可。

SELECT
    engineer_id,
    COUNT(*) AS request_count,
    SUM(TIMESTAMPDIFF(SECOND, created_at, assigned_at)) AS total_time_taken,
    SUM(TIMESTAMPDIFF(SECOND, created_at, assigned_at)) /
    COUNT(*) AS average
FROM requests
WHERE deleted_at IS NULL AND
      submitted_at >= '2017-03-30 00:00:00' AND
      deleted_at IS NULL
GROUP BY engineer_id
LIMIT 5