Calculations of different columns in Mysql Query - mysql

Question

I have a Table:-
+-----+--------------+--------------+----------+--------------------+---------------+-----------------+
| id | CustomerName | VideoQuality | IsActive | BufferedTime | ElapsedTime | TotalBufferTime |
+-----+--------------+--------------+----------+--------------------+---------------+-----------------+
| 139 | HotStar | 180 | Yes | 10.367167126617211 | 30.000000000 | NULL |
| 140 | HotStar | 1300 | NULL | 5.43524230876729 | 34.000000000 | NULL |
| 141 | HotStar | 1300 | NULL | 5.671054515212042 | 38.000000000 | NULL |
| 142 | HotStar | 1300 | NULL | 5.045639532902047 | 41.000000000 | NULL |
| 143 | HotStar | 1300 | NULL | 5.455747718023355 | 44.000000000 | NULL |
| 144 | HotStar | 1300 | NULL | 5.691559924468107 | 49.000000000 | NULL |
i want to calculate the columns BufferTime and ElapsedTime and insert that output to TotalBufferTime column but i want to skip the first row of the BufferTime.
So the fisrt calculation will be 5.43 + 30.000 second calculation will be 5.67 + 34.00 and so on.
I also have a column IsActive which shows the first row of Buffer time.
I want to do something like this :-
update RequestInfo SET `TotalBufferTime` = BufferedTime + ElapsedTime;
only thing i want to skip only the first row of the column buffered time.

Answer 1

Assuming you a field id that determines row order in your table, you can use a correlated subquery so as to get BufferedTime of previous row like this:
SELECT t1.CustomerName, t1.VideoQuality, t1.IsActive, t1.BufferedTime,
t1.ElapsedTime,
(SELECT t2.BufferedTime
FROM mytable AS t2
WHERE t2.td > t1.id
ORDER BY id LIMIT 1) + t1.ElapsedTime AS TotalBufferTime
FROM mytable AS t1
WHERE IsActive IS NULL
Edit:
To UPDATE you can use the following query:
SET #et = 0;
SET #ElapsedTime = NULL;
UPDATE RequestInfo
SET TotalBufferTime = CASE
WHEN (#et := #ElapsedTime) < 0 THEN NULL
WHEN #ElapsedTime := ElapsedTime THEN BufferedTime + #et
END
ORDER BY id;
The trick here is to use a CASE expression where the first WHEN clause is always evaluated (because it is the first one) but is never true. This way #et variable is initialized with the value of #ElapsedTime, i.e. the value of the previous record.
Demo here