MySQL TIMEDIFF（）返回两次之间的差值和时间之和

我的问题是，我想用timediff函数查找总和。有什么帮助吗

SELECT TIMEDIFF( timefrom, timeto )
FROM `leaveapply`
WHERE userid = '21'
GROUP BY leavetype
LIMIT 0 , 30

下面的语句将返回如下时间

00:00:00
00:00:00
01:00:00
02:00:00

我想求和得到结果，

---

像这样的

03:00:00

试试这样：

select sum(
            SELECT TIMEDIFF( timefrom, timeto ) FROM `leaveapply`
            WHERE userid = '21'
            GROUP BY leavetype
          ) as total