LAG: which row does it look back to?

Question

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.