SQL fundamentals interview questions

Table products(id, category, price) where price is never NULL. Return one row per category with category, product_count (how many products it has) and total_price (the sum of their prices). Categories are compared exactly as stored.

Table orders(id, customer_id), customer_id never NULL. Return customer_id and order_count for every customer who has placed three or more orders. Customers below the threshold must not appear at all.

Table reviews(id, product_id, rating) where rating is an integer 1-5 that is NULL when the reviewer left only text. For every product that has at least one review return product_id, review_count (all reviews), rated_count (reviews that carry a rating) and avg_rating (the average

Table tickets(id, team, status) where status is free text but the report only cares about 'open', 'closed' and 'blocked'. Return one row per team that has at least one ticket, with team, open_count, closed_count and blocked_count. A team with no tickets in a given

Walk me through the difference between COUNT(*), COUNT(price) and COUNT(DISTINCT price) on the same table. On a table of five rows where two prices are missing and the other three are all 9.99, what does each one return?

What does GROUP BY actually do to a result set? And why does the database reject SELECT category, name, SUM(price) FROM products GROUP BY category?