How to sum the hour of time based on same id with mysql

I never done a time summary before. But the idea is to sum all the time in hour from DB. Based on the same id and time period. For the sample data below. I want to know how many hour in which id '57' working in month 4. Only when there're starting time(chkin_type=1) and ending time (chkin_type=2).

So I know that I have to find the time diff with
DATEDIFF(hour, time1, time2). But I don't know how to loop through the last row.

Sample data

I can do the select.

select * from checkin_db where uid='57' and month(dtime)='4'


But I stuck here:

1. How to select dtime where chkin_type=1 to compare with chkin_type=2
2. How to loop the result and sum the total time in hours with datediff?

The data has to be pivoted and then it is possible to calculate the time differences and then sum them.
Since MySql lacks pivot functionality it must be simulated using CASE WHEN together with an aggregate function (in this case max())

So the inner query 'pvt' selects the wanted data and pivots it.
The second-level query 'mins' calculates the time difference in minutes for each day and sums the results.
Finally the outer query splits the time in hour and minutes.

Keep in mind that all this can work only if the two time values are in the same day.

SELECT total_minutes DIV 60 as hours, total_minutes MOD 60 as minutes
FROM (

	SELECT sum(timestampdiff(MINUTE, time1, time2)) as total_minutes
	FROM (

		SELECT date(dtime) as date,
			   max(CASE WHEN ckin_type = 1 THEN time(dtime) ELSE NULL END) as time1,
			   max(CASE WHEN ckin_type = 2 THEN time(dtime) ELSE NULL END) as time2
		FROM checkin_db
		WHERE uid='57' and month(dtime)='4'
		GROUP BY date(dtime)
	
	) pvt
	
   WHERE time1 IS NOT NULL
     AND time2 IS NOT NULL
	 
) mins;