caculating while select - maybe sum ?

Hi All

im trying to calculate hour usage so a typical row is: 1. enter time, 2.
exit time 3. sum() until this row

Couldn't use sum() as it ask me to 'group by' which I don't want.

Is there a way to add the previous row value to the current, per row ?

example:
select row1, row2, (row1 + row2 until now) from table;

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