SQL：计算行之间差异的平均值

Question

I have a table like below

session   stepId        starttime     
------   -----------    -----    
1        1              10:00        
1        1              10:10        
1        2              10:40        
1        3              10:50        
1        4              11:00

And what I am aiming to calculate is the average time between each step Id, if the stepID is the same , like the first two rows, the most recent one is used. For example, for the above query, the result should be ((10:40 - 10:10) + (10:50-10:40) + (11:00 - 10:50))/3. I am using MySQL.

Answer 1

SELECT 
    TIME(AVG(M.timediff))
FROM
    (SELECT 
        TIME(b.starttime - a.starttime) AS timediff
    FROM
        (SELECT 
        stepId, MAX(starttime) starttime
    FROM
        test.test
    GROUP BY stepId) a
    LEFT JOIN test.test b ON a.stepId = b.stepId - 1
    WHERE
        a.starttime IS NOT NULL
            AND b.starttime IS NOT NULL) AS M