Mysql timediff返回兩個不同的值

Question

我有一個查詢，我正在測試時間。

我得到兩個不同的時間，這使我懷疑我的數據的正確性。

這裏有兩個例子：

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; 

這將返回：

00:01:17 and 00:00:05 

下一個查詢是：

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 

爲什麼會出現這種情況和結果集是我應該去相信？如果任何人能夠清除我的困惑，會很樂意。

Answer 1

TIMEDIFF（）返回一個字符串，例如：

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

如果直接把這個費用分攤，MySQL將嘗試投04:01:56爲int，並與avg(04)，而不是結束。

這就是爲什麼你必須time_to_sec，它返回的時間值作爲一個簡單的INT：14516，可直接取平均值。