How to translate from SQL to NoSQL/MapReduce?

I have a background working with relational databases but recently started to dabble in CouchDB and was surprised by how some non-relational operations, which would be simple in SQL, were not first-class functions in CouchDB.

I would appreciate you taking a moment to map each SQL statement below to its MapReduce equivalent.

SELECT COUNT(*) FROM products WHERE price < 20.00;
SELECT category, SUM(price) FROM products GROUP BY category;
UPDATE products SET price = 19.99 WHERE price = 20.00;
DELETE FROM products WHERE expires_at <= NOW();