PostgreSQL | Count with unnest

-- https://platform.stratascratch.com/coding/10049-reviews-of-categories
/* Data:

    select categories, review_count
    from yelp_business
    limit 3

categories                                                                                    | review_count
----------------------------------------------------------------------------------------------|-------------
Auto Detailing;Automotive                                                                       4
Personal Chefs;Food;Gluten-Free;Food Delivery Services;Event Planning & Services;Restaurants    27
Dry Cleaning & Laundry;Laundry Services;Local Services;Dry Cleaning                             4
*/

select
    unnest(string_to_array(categories,';')),
    sum(review_count)
from yelp_business
group by 1
order by 2 desc