MySQL GroupBy with null/zero results - mysql

Question

I'm currently writing a ticket system that has three tables
one for users:
users
+----+-----------+----------+
| ID | FirstName | LastName |
+----+-----------+----------+
| 1 | First | User |
| 2 | Second | User |
| 3 | Third | User |
| 4 | Fourth | User |
| 5 | Fifth | User |
+----+-----------+----------+
one for tickets:
ticket
+----+---------------+
| ID | TicketSubject |
+----+---------------+
| 1 | Ticket #1 |
| 2 | Ticket #2 |
| 3 | Ticket #3 |
| 4 | Ticket #4 |
+----+---------------+
and one to assign users to tickets to action (can be more than one user per ticket):
ticket_assigned
+----+----------+--------+
| ID | TicketID | UserID |
+----+----------+--------+
| 1 | 1 | 1 |
| 2 | 1 | 2 |
| 3 | 2 | 1 |
| 4 | 3 | 5 |
| 5 | 3 | 3 |
+----+----------+--------+
I'm trying to create a summary to show each user, and how many tickets they have assigned to them, example:
+------------+-------+
| Name | Count |
+------------+-------+
| First | 2 |
| Second | 1 |
| Third | 1 |
| Fourth | 0 |
| Fifth | 1 |
| Unassigned | 2 |
+------------+-------+
Note that the last entry is "unassigned", this is the number of records in the ticket table that DONT appear in the ticket_assigned table (thus being, unassigned). Also further note that user "Fourth" is zero, in that that user has no records in the ticket_assigned table.
Here is the current MySQL query I am using:
SELECT
CASE
WHEN users.FirstName IS NULL
THEN 'Unassigned'
ELSE users.FirstName
END as 'UserName',
COUNT(*) as 'TicketCount'
FROM tickets
LEFT OUTER JOIN ticket_assigned ON tickets.ticket_id = ticket_assigned.ticket_id
LEFT OUTER JOIN users ON ticket_assigned.user_id = users.user_id
GROUP BY ticket_assigned.user_id
ORDER BY UserName;
Problem with this is that it's not showing any of the users that don't feature in the ticket_assigned table, I'm essentially getting this:
+------------+-------+
| Name | Count |
+------------+-------+
| First | 2 |
| Second | 1 |
| Third | 1 |
| Fifth | 1 |
| Unassigned | 2 |
+------------+-------+
Is anyone able to assist and tell me how I can modify my query to include users that have no records in the ticket_assigned table? Thanks in advance!

Answer 1

Use a LEFT JOIN with a subquery to aggregate tickets:
SELECT t1.FirstName,
COALESCE(t2.ticket_count, 0) AS num_tickets
FROM users t1
LEFT JOIN
(
SELECT UserID, COUNT(*) AS ticket_count
FROM ticket_assigned
GROUP BY UserID
) t2
ON t1.ID = t2.UserID
UNION ALL
SELECT 'Unassigned', COUNT(*)
FROM tickets t
WHERE NOT EXISTS (SELECT 1 FROM tickets_assigned ta
WHERE ta.ticketId = t.id)

Answer 2

In MySQL, I think you need a left join and union all:
select u.id, u.firstname, count(ta.userId) as num_tickets
from users u left join
tickets_assigned ta
on ta.userId = u.id
group by u.id, u.firstname
union all
select NULL, 'Unassigned', count(*)
from tickets t
where not exists (select 1
from tickets_assigned
where ta.ticketId = t.id
);
I included the u.id in the aggregations. I'm uncomfortable just aggregating (and reporting) by first name, because different people frequently have the same first name, even in a relatively small group.

Answer 3

SELECT
u2.Firstname, IFNULL(tmp.count, 0) AS count
FROM users u2
LEFT JOIN (
SELECT u.id, u.Firstname, COUNT(1) as count
FROM ticket_assigned ta
LEFT JOIN ticket t ON t.id = ta.ticketID
LEFT JOIN users u ON u.id = ta.userID
GROUP BY u.id
) tmp ON tmp.id = u2.id
UNION
SELECT
'Unassigned', count(1) AS count
FROM ticket
WHERE id NOT IN (SELECT ticketid FROM ticket_assigned)