How to add an extra column that is the cumulative value of the time differences for each course? For example, the initial table is:
id_A course weight ts_A value
id1 cotton 3.5 2017-04-27 01:35:30 150.000000
id1 cotton 3.5 2017-04-27 01:36:00 416.666667
id1 cotton 3.5 2017-04-27 01:36:30 700.000000
id1 cotton 3.5 2017-04-27 01:37:00 950.000000
id2 cotton blue 5.0 2017-04-27 02:35:30 150.000000
id2 cotton blue 5.0 2017-04-27 02:36:00 450.000000
id2 cotton blue 5.0 2017-04-27 02:36:30 520.666667
id2 cotton blue 5.0 2017-04-27 02:37:00 610.000000
The expected result is:
id_A course weight ts_A value cum_delta_sec
id1 cotton 3.5 2017-04-27 01:35:30 150.000000 0
id1 cotton 3.5 2017-04-27 01:36:00 416.666667 30
id1 cotton 3.5 2017-04-27 01:36:30 700.000000 60
id1 cotton 3.5 2017-04-27 01:37:00 950.000000 90
id2 cotton blue 5.0 2017-04-27 02:35:30 150.000000 0
id2 cotton blue 5.0 2017-04-27 02:36:00 450.000000 30
id2 cotton blue 5.0 2017-04-27 02:36:30 520.666667 60
id2 cotton blue 5.0 2017-04-27 02:37:00 610.000000 90