的MySQL的GroupBy與空/零點導致

Question

我目前正在寫有三個表

一個爲用戶提供客票系統：

users 
    +----+-----------+----------+ 
    | ID | FirstName | LastName | 
    +----+-----------+----------+ 
    | 1 | First  | User  | 
    | 2 | Second | User  | 
    | 3 | Third  | User  | 
    | 4 | Fourth | User  | 
    | 5 | Fifth  | User  | 
    +----+-----------+----------+ 

一個門票：

ticket 
    +----+---------------+ 
    | ID | TicketSubject | 
    +----+---------------+ 
    | 1 | Ticket #1  | 
    | 2 | Ticket #2  | 
    | 3 | Ticket #3  | 
    | 4 | Ticket #4  | 
    +----+---------------+ 

和一個爲用戶指定動作票（可以每票多於一位用戶）：

ticket_assigned 
    +----+----------+--------+ 
    | ID | TicketID | UserID | 
    +----+----------+--------+ 
    | 1 |  1 |  1 | 
    | 2 |  1 |  2 | 
    | 3 |  2 |  1 | 
    | 4 |  3 |  5 | 
    | 5 |  3 |  3 | 
    +----+----------+--------+ 

我試圖創建摘要顯示每個用戶，以及他們有多少票分配給他們，例如：

+------------+-------+ 
| Name | Count | 
+------------+-------+ 
| First  |  2 | 
| Second  |  1 | 
| Third  |  1 | 
| Fourth  |  0 | 
| Fifth  |  1 | 
| Unassigned |  2 | 
+------------+-------+ 

注意，最後一項是「未分配」，這是多少記錄在故障單表中，不要出現在ticket_assigned表中（因此，未分配）。另請注意，用戶「Fourth」爲零，因爲該用戶在ticket_assigned表中沒有記錄。

這裏是我使用當前MySQL查詢：

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; 

問題的是，它沒有顯示，我基本上得到這是任何不中ticket_assigned表功能的用戶：

+------------+-------+ 
| Name | Count | 
+------------+-------+ 
| First  |  2 | 
| Second  |  1 | 
| Third  |  1 | 
| Fifth  |  1 | 
| Unassigned |  2 | 
+------------+-------+ 

是否有人能夠協助並告訴我如何修改我的查詢以包含在ticket_assigned表中沒有記錄的用戶？提前致謝！

Answer 1

使用帶有子查詢LEFT JOIN到總門票：

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

在MySQL中，我認爲你需要一個left join和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 
       ); 

我包含在聚合的u.id。我不願意通過名字來聚合（和報告），因爲不同的人經常使用相同的名字，即使是在一個相對較小的羣體中。

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)