Displaying the value with the most occurrences - mysql

Question

I have two related tables:
(1) people contains names and image files.
(2) cities contains cities they have visited.
people
id name image
1 John NULL
2 Carrie 001.jpg
3 Desmond 002.jpg
4 Harry 003.jpg
5 Paul NULL
cities
id city people_id year_visited
1 Chicago 1 2000
2 Chicago 4 2000
3 Chicago 5 2001
4 Paris 1 2000
5 Paris 2 2002
6 Chicago 4 2002
7 Chicago 1 2001
8 London 1 2004
9 Sydney 5 2001
10 Sydney 1 2002
11 Rio 5 2002
12 London 5 2004
13 Sydney 5 2003
14 Sydney 5 2005
I would like to identify all people without an image, and the city they have visited the most. So the results I am looking for is:
name most_visited_city number_of_visits
John Chicago 2
Paul Sydney 3
I can group_concat the cities they have visited, but not drill down to the single city they visited the most.
All help gratefully appreciated.

Answer 1

The following gets people, cities, and the count:
select p.id, c.city, count(*) as cnt
from people p join
cities c
on p.id = c.people_id
where p.image is null
group by p.id, c.city;
Getting information about the most visited is tricky in MySQL. Here is one method that works if the data is not too large:
select id,
substring_index(group_concat(city order by cnt desc separator '|'), '|', 1) as most_visited_city,
max(cnt) as number_of_times_visited
from (select p.id, c.city, count(*) as cnt
from people p join
cities c
on p.id = c.people_id
where p.image is null
group by p.id, c.city
) pc
group by id;

Answer 2

This query should return the most visited city for each people_id in cities.
SELECT t1.people_id, t2.city, t2.visits
FROM (
SELECT people_id, MAX(visits) AS max_visits
FROM (
SELECT people_id, city, COUNT(*) AS visits
FROM cities
GROUP BY people_id, city) x
GROUP BY people_id) AS t1
JOIN (
SELECT people_id, city, COUNT(*) AS visits
FROM cities
GROUP BY people_id, city) AS t2
ON t1.people_id = t2.people_id AND t1.max_visits = t2.visits
The general structure is based on an answer in SQL Select only rows with Max Value on a Column, but instead of getting the max value of a column in the table, it's using the max value in the subquery that counts visits per city. Unfortunately, it results in an ugly query because you have to repeat that subquery, since MySQL doesn't have CTEs.
Then you can join it with people to get the person's name and filter out the ones with an image.
SELECT p.name, t2.city, t2.visits
FROM (
SELECT people_id, MAX(visits) AS max_visits
FROM (
SELECT people_id, city, COUNT(*) AS visits
GROUP BY people_id, city) x
GROUP BY people_id) AS t1
JOIN (
SELECT people_id, city, COUNT(*) AS visits
GROUP BY people_id, city) AS t2
ON t1.people_id = t2.people_id AND t1.max_visits = t2.max_visits
JOIN people AS p ON p.id = t1.people_id
WHERE p.image IS NULL
DEMO