Question

我目前正在编写一张有三张桌子的票务系统

一个用户：

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

一张票：

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

和一个为用户分配行动票（每张票可以多个用户）：

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

我试图创建一个摘要来显示每个用户，以及他们分配给他们的票数，例如：

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

请注意，最后一个条目是＆＃34;未分配＆＃34;，这是故障单表中DONT出现在ticket_assigned表中的记录数（因此，未分配）。还要注意用户＆＃34; Fourth＆＃34;为零，因为该用户在ticket_assigned表中没有记录。

以下是我正在使用的当前MySQL查询：

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;

问题在于，它没有显示在ticket_assigned表中没有任何功能的用户，我基本上得到了这个：

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

是否有人能够协助并告诉我如何修改我的查询以包含ticket_assigned表中没有记录的用户？提前谢谢！

Answer 1

使用带有子查询的LEFT JOIN汇总票证：

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，我认为您需要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
                 );

我在汇总中加入了u.id。我只是通过名字汇总（和报告）而感到不舒服，因为不同的人经常拥有相同的名字，即使是在相对较小的群体中也是如此。

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)

MySQL GroupBy的结果为null / 0

3 个答案: