Summing field in other rows conditionally - sql

Question

I have table in the form like below:
Pilot
Leg
Duration
Takeoff
John
1
60
9:00:00
John
2
60
9:00:00
John
3
30
9:00:00
Paul
1
60
12:00:00
Paul
2
30
12:00:00
Paul
3
30
12:00:00
Paul
4
60
12:00:00
And I am trying to figure out is a query to get the following:
Pilot
Leg
Duration
Takeoff
LegStart
John
1
60
9:00:00
9:00:00
John
2
60
9:00:00
10:00:00
John
3
30
9:00:00
10:30:00
Paul
1
60
12:00:00
12:00:00
Paul
2
30
12:00:00
13:00:00
Paul
3
30
12:00:00
13:30:00
Paul
4
60
12:00:00
14:00:00
So the 'LegStart' time is the 'TakeOff' time, plus the duration of prior legs for that pilot.
Now , to do this in SQL, I need to somehow add up the durations of prior legs for the same pilot. But for the life of me... I cannot figure out how you can do this because the pilots can have a variable number of legs, so joining doesn't get you anywhere.

Answer 1

You can use a cumulative sum. The trick is including this in the
select t.*,
sum(duration) over (partition by pilot order by leg) as running_duration,
datetime_add(takeoff,
interval (sum(duration) over (partition by pilot order by leg) - duration) minute
) as leg_start
from t;
Note: This assumes that takeoff is a datetime.

Answer 2

Try analytic SUM sum(duration) over (partition by pilot order by leg):
with mytable as (
select 'John' as pilot, 1 as leg, 60 as duration, time '9:00:00' as takeoff union all
select 'John', 2, 60, '9:00:00' union all
select 'John', 3, 30, '9:00:00' union all
select 'Paul', 1, 60, '12:00:00' union all
select 'Paul', 2, 30, '12:00:00' union all
select 'Paul', 3, 30, '12:00:00' union all
select 'Paul', 4, 60, '12:00:00'
)
select
*,
time_add(takeoff, interval ifnull(sum(duration) over (partition by pilot order by leg ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING), 0) minute) as legstart
from mytable