Answer
LAG returns a value from an earlier row in the window ordering. • An offset chooses how many rows back to read. • A default can be supplied when no earlier row exists. • It is useful for change calculations and previous-row comparisons.
💡 SQL Example
SELECT ordered_at, amount, LAG(amount) OVER (ORDER BY ordered_at) AS previous_amount FROM payments;
Result
Each payment with the previous payment amount
⚡ Quick Revision
LAG reads a value from an earlier ordered row.