MySQL get pairs of rows from same set of foreign table - mysql

Question

My issue is the next one.
I have 3 tables: People, Cars and Driven:
People:
Id Name
1 | Tom
2 | James
3 | Charles
4 | Eric
5 | Thomas
6 | Robert
7 | Kim
8 | Ellias
Cars:
Id Name
1 | Ford
2 | Nissan
3 | Hyundai
Driven:
PID CID (People ID & Car ID)
1 | 1
2 | 1
5 | 1
5 | 2
6 | 1
6 | 2
7 | 1
7 | 2
7 | 3
8 | 1
I Want to retrieve pairs of people that driven the SAME SET OF CARS.
I mean: if Tom driven only Ford and James driven also ONLY Ford, i want to return this pair Tom/James as result. Also i want to include pairs of people that didn't driven any car (ie. Charles/Eric (0 cars driven both)).
The query result with the example above should return two columns per result, for example:
Name Name
Tom | James (Only Ford)
Tom | Ellias (Only Ford)
James | Ellias (Only Ford)
Charles | Eric (None BOTH)
Thomas | Robert (Ford and Nissan BOTH)
Also notice that Kim has driven Ford, Nissan and Hyundai. So Kim is not going to be pair with anybody. Tom James and Ellias all are driven Ford, so they are pair with themselves.
I'm tried with cartesian product and relational division, but I didn't find a solution. If someone can help me at least with a tip i will be really grateful. Thanks!

Answer 1

You can use the following query:
SELECT p.Id, p.Name,
COALESCE(GROUP_CONCAT(c.Name ORDER BY c.Name), 'None') AS cars_driven
FROM People AS p
LEFT JOIN Driven AS d ON p.Id = d.PID
LEFT JOIN Cars AS c ON c.Id = d.CID
GROUP BY p.Id, p.Name;
to get the list of cars driven per person.
Output:
Id Name cars_driven
-----------------------
1 Tom Ford
2 James Ford
3 Charles None
4 Eric None
5 Thomas Ford,Nissan
6 Robert Ford,Nissan
7 Kim Ford,Hyundai,Nissan
8 Ellias Ford
Using the above query twice as a derived table you can get the required result:
SELECT t1.Name, t2.Name, t1.cars_driven
FROM (
SELECT p.Id, p.Name,
COALESCE(GROUP_CONCAT(c.Name ORDER BY c.Name), 'None') AS cars_driven
FROM People AS p
LEFT JOIN Driven AS d ON p.Id = d.PID
LEFT JOIN Cars AS c ON c.Id = d.CID
GROUP BY p.Id, p.Name) AS t1
JOIN (
SELECT p.Id, p.Name,
COALESCE(GROUP_CONCAT(c.Name ORDER BY c.Name), 'None') AS cars_driven
FROM People AS p
LEFT JOIN Driven AS d ON p.Id = d.PID
LEFT JOIN Cars AS c ON c.Id = d.CID
GROUP BY p.Id, p.Name
) AS t2 ON t1.Id < t2.Id AND t1.cars_driven = t2.cars_driven;
Output:
Name Name cars_driven
----------------------------
Tom James Ford
Charles Eric None
Thomas Robert Ford,Nissan
Tom Ellias Ford
James Ellias Ford
Demo here