Windowing ?

Hi,

I have a table describing sort of ordered road segments and their associated
weight. I would like to calculate weight deltas between two consecutive
segments. Problem is to substract previous from current row and reuses
result as input for the next row. I tried some windowing tests but was
unsuccessful.

Input table :

road segment weight

1 1 80

1 2 5

1 3 3

2 1 75

2 2 8

2 3 12

2 4 3

The result should be a new delta column :

road segment weight delta

1 1 80 80 (80-0) first row

1 2 5 75 (80-5)

1 3 3 72 (75-3)

2 1 75 75 (75-0)

2 2 8 67 (75-8)

2 3 12 55 (67-12)

2 4 3 52 (55-3)

Thanks for any help,

By the way, thanks again to Andreas and Martin that unstucked my mind
yesterday as I was searching for complicated solutions when a very simple
one existed.

Have a good day (or good evening)

Olivier

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