Question

我有一个查询，我正在测试时间。

我有两次不同的时间，这让我怀疑数据的正确性。

以下是两个例子：

SELECT 
    users.fname as counselor,
    count(session.anum) as students,
    SEC_TO_TIME(AVG(TIME_TO_SEC(TIMEDIFF(starttime, signintime)))) AS 'AVG Wait Time',
    SEC_TO_TIME(AVG(TIME_TO_SEC(TIMEDIFF(finishtime, starttime)))) AS 'AVG Session Length'
FROM session
    LEFT JOIN support 
        ON support.session_id = session.session_id
    LEFT JOIN users 
        ON users.username = support.counselor
WHERE session.status = 3 
GROUP BY fname;

返回：

00:01:17 and 00:00:05

下一个查询是：

SELECT 
    users.fname as counselor,
    count(session.anum) as students,
    SEC_TO_TIME(AVG(TIMEDIFF(starttime, signintime))) AS 'AVG Wait Time',
    SEC_TO_TIME(AVG(TIMEDIFF(finishtime, starttime))) AS 'AVG Session Length'
FROM session
    LEFT JOIN support 
        ON support.session_id = session.session_id
    LEFT JOIN users 
        ON users.username = support.counselor
WHERE session.status = 3 
GROUP BY fname;

00:01:37 and 00:00:05

为什么会发生这种情况以及我应该相信哪种结果？如果有人能够清除我的困惑，那就爱。

Answer 1

TIMEDIFF（）返回一个字符串，例如

TIMEDIFF('13:07:33', '9:5:37') -> 04:01:56

如果您直接对其进行平均，则mysql会尝试将04:01:56转换为int并最终使用avg(04)。

这就是为什么你必须time_to_sec，它将该时间值作为简单的int返回：14516，可以直接平均。

Mysql timediff返回两个不同的值

1 个答案: