嵌套案例：我怎么能正确写？

Question

res

逻辑是以防万一！我怎么写得好？ 我一般可以这样写吗？

Answer 1

CASE    WHEN book_id is not null
    THEN CASE 
                    WHEN title=title_id
                    THEN title_id
                    ELSE 0 
         END,
    ELSE CASE 
                    WHEN "Handle the other cases for book"
                    THEN "Case 2"
                    ELSE "Default for other Case" 
         END
END