That's it! In part 6, we introduced analytic functions: LEAD, LAG, FIRST_VALUE, LAST_VALUE, NTH_VALUE. Take a look:
SELECT id, rating, LAG(rating) OVER(ORDER BY id) FROM review;
The example above shows the rating given in each review and the rating given in the previous review.
For each giftcard, show its amount_worth, payment_amount and two more columns: the payment_amount of the first and last giftcards purchased in terms of the payment_date.
Use both FIRST_VALUE and LAST_VALUE. Remember that LAST_VALUE requires redefining the window frame:
ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
