Ranking rows

ROW_NUMBER, RANK, and DENSE_RANK.

Number and rank

Ranking functions need an ORDER BY inside the window:

ROW_NUMBER() — 1,2,3… a unique position
RANK() — ties share a rank, then it skips (1,1,3)
DENSE_RANK() — ties share a rank, no gaps (1,1,2)

Most expensive book per genre — number them within each genre, priciest first:

SELECT title, genre, price,
       RANK() OVER (PARTITION BY genre ORDER BY price DESC) AS price_rank
FROM books;

The restock list, ORDER BY stock alone. Two books hold zero, and that tie is the only place the three functions disagree: ROW_NUMBER invents an order (which of the two gets 1 is arbitrary), RANK leaves a gap where the tie consumed a position, DENSE_RANK does not.

RANK() OVER (PARTITION BY genre ORDER BY price DESC). The partition is the reset: rank 1 means "priciest in its genre", never "priciest overall".