Meta Data Engineer interview questions

Calculate the average book price from a list of prices. 2. Determine the maximum number of unique books that can be bought, given an array of book prices and a budget (each book can be purchased only once). 3. From a given list of book titles, identify sequels (e.g., for an input like ['GOT', 'Troy', 'Batman', 'Batman Returns'], the output should be ['Batman Returns'])

Which is more efficient? UNION or UNION ALL?