MySQL: Find rows in one table and not the other, find both - mysql

Question

I'm struggling to get my head around MySQL joins. I have three tables
-- events
id name
1 Event 1
2 Event 2
-- registrations
id event name
1 1 Alice
2 1 Bob
3 2 Alice
4 2 Charlie
-- scores
id event name score
1 1 Alice 10
2 1 Charlie 20
3 2 Alice 15
4 2 Bob 30
For each event I'm trying to work out
How many people registered (rows in registration table) but did NOT get a score (exclude rows in scores table)
How many people got a score (rows in scores table) but did NOT register (exclude rows in registration table)
How many people BOTH registered and got a score
I've tried different variations of
SELECT *
FROM registrations r
LEFT JOIN scores s
ON r.event = s.event
WHERE s.event IS NULL
AND r.event = 1
but I'm not sure what I should be joining on: event or name but neither are null and I never seem to get the correct numbers I'm looking for. The result at the end should be like
name reg_only score_only reg&score total
event Event 1 1 1 1 3

Answer 1

You can use below 3 queries in same sequence as you mentioned in your query-
SELECT e.id, e.name, COUNT(r.id) AS registered_user
FROM `events` AS e
INNER JOIN registrations AS r ON e.id=r.event
LEFT JOIN scores AS s ON e.id = s.event
WHERE s.event IS NULL;
SELECT e.id, e.name, COUNT(r.id) AS scored_user
FROM `events`scores AS e
INNER JOIN scores AS s ON e.id=s.event
LEFT JOIN registrations AS r ON e.id = r.event
WHERE r.event IS NULL;
SELECT e.id, e.name, COUNT(*) AS both_user
FROM `events` AS e
INNER JOIN registrations AS r ON e.id=r.event
INNER JOIN scores AS s ON e.id = s.event;