Design A: one table for common elements and N tables for uncommon elements
    Table Product
        int product_id
        string brand_name
        number price
        number weight
        int product_type        #1 for fish, 2 for bicycles
        int fish_or_bike_id     #foreign key into either the fish table or bicycle table

    Table Fish
        int fish_id
        bool requires_salt_water

    Table Bicycle
        int bicycle_id
        int number_of_spokes

Design B: everything in one table, even if it doesn't make sense
    Table Product
        int product_id
        string brand_name
        number price
        number weight
        int product_type        #1 for fish, 2 for bicycles
        bool requires_saltwater #always False for bicycles
        int number_of_spokes    #always 0 for fish

Design C: no common tables whatsoever
    Table Fish
        int product_id
        string brand_name
        number price
        number weight
        bool requires_salt_water
    Table Bicycle
        int product_id
        string brand_name
        number price
        number weight
        int number_of_spokes