LAG to access the previous row

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Accepted Answer

LAG(amount) returns the value of amount from the previous row in the window order. Ordered by month (Jan, Feb, Mar), Feb's previous row is Jan with amount=100. Jan has no previous row, so LAG returns NULL for Jan. LAG is perfect for period-over-period comparisons: amount - LAG(amount) OVER (ORDER BY month) gives the month-over-month change.