MySQL Join Group By 和 Group Concat

Question

我正在跟踪连接到网络应用程序并在问题提要中留下问题的用户

用户

id  name
---------
1   Jim
2   Joe

因为用户可以从各种浏览器连接到应用程序并且有多个连接，所以我使用 Token 表来跟踪用户连接：

代币

id  userid
----------
1   1
2   1
3   2
4   2

然后用户提出问题：

问题

id  userid  pageno text
------------------------------------------------
1   1       1      'Whats going on here?'
2   1       2      'How are you?'
4   2       2      'Good and you?'

我的目标是输出以下内容

username  questions                               Questions Page 1        Questions Page 2
-----------------------------------------------------------------------------------
Jim       'Whats going on here?', 'How are you?'  'Whats going on here?'  'How are you?'
Joe       'Good and you?'                                                 'Good and you?'

现在我尝试直接查询用户表，但由于我的数据库中有很多用户，我需要查询令牌表而不是用户表以获得性能，然后按用户 ID 分组，因为一个用户可以获得许多令牌。

虽然现在我按用户 ID 分组，但我不确定如何对不同的问题进行分组。 查看查询：

SELECT  u.id, u.name, q.text
FROM token t
LEFT JOIN user as u ON t.userid = u.id
LEFT JOIN question as q ON t.userid = q.userid
GROUP BY t.userid

我的问题是我只得到了第一个问题，但我尝试将 concat 分组但没有运气。

忘记补充了，如果可以动态完成，那就太好了，因为页码会发生很大变化 你能帮忙吗？ 谢谢

Answer 1

尝试了GROUP_CONCAT(DISTINCT q.text) as question FROM之后你得到了什么？

SELECT  u.id, u.name, GROUP_CONCAT(DISTINCT q.text) as question
FROM token t
LEFT JOIN user as u ON t.userid = u.id
LEFT JOIN question as q ON t.userid = q.userid
GROUP BY t.userid

Answer 2

查询应该是这样的：

SELECT temp.*, (SELECT GROUP_CONCAT(q.text) FROM questions q WHERE q.pageno = temp.pageno) as Pagewiseqtn FROM (SELECT  u.id, u.name, GROUP_CONCAT(DISTINCT q.text) as question, q.pageno
FROM tokens t
LEFT JOIN users as u ON t.userid = u.id
LEFT JOIN questions as q ON t.userid = q.userid
GROUP BY t.userid) as temp

输出：

Answer 3

获得 OP 要求的确切结果的最容易理解和最简单的方法是将问题划分为多个子查询而不是单个查询。

我创建了多个中间VIEW 来获得最终结果。此外，使用中间 VIEW 而不是中间 TABLE 的好处是原始表中的任何更新也会导致 VIEW 中的结果更新

子查询 1：

CREATE VIEW q AS
    -> SELECT userid,  
    -> GROUP_CONCAT(text ORDER BY userid SEPARATOR ", ") AS questions
    -> FROM Questions
    -> GROUP BY userid;
    
SELECT * FROM q;
+--------+----------------------------------------+
| userid | questions                              |
+--------+----------------------------------------+
|      1 | 'How are you?', 'Whats going on here?' |
|      2 | 'Good and you?'                        |
+--------+----------------------------------------+

子查询 2：

CREATE VIEW q1 AS
    -> SELECT userid, text AS questions1
    -> FROM Questions
    -> WHERE pageno = 1;

SELECT * FROM q1;
+--------+------------------------+
| userid | questions1             |
+--------+------------------------+
|      1 | 'Whats going on here?' |
+--------+------------------------+

子查询 3：

CREATE VIEW q2 AS
    -> SELECT userid, text AS questions2
    -> FROM Questions
    -> WHERE pageno = 2;
    
SELECT * FROM q2;
+--------+-----------------+
| userid | questions2      |
+--------+-----------------+
|      1 | 'How are you?'  |
|      2 | 'Good and you?' |
+--------+-----------------+

子查询 4：

CREATE VIEW ql AS
    -> SELECT u.id, q.questions
    -> FROM Users AS u
    -> LEFT OUTER JOIN
    -> q ON u.id = q.userid;
    
SELECT * FROM ql;
+----+----------------------------------------+
| id | questions                              |
+----+----------------------------------------+
|  1 | 'How are you?', 'Whats going on here?' |
|  2 | 'Good and you?'                        |
+----+----------------------------------------+

子查询 5：

CREATE VIEW q1l AS
    -> SELECT u.id, q1.questions1
    -> FROM Users AS u
    -> LEFT OUTER JOIN
    -> q1 ON u.id = q1.userid;

SELECT * FROM q1l;
+----+------------------------+
| id | questions1             |
+----+------------------------+
|  1 | 'Whats going on here?' |
|  2 | NULL                   |
+----+------------------------+

子查询 6：

CREATE VIEW q2l AS
    -> SELECT u.id, q2.questions2
    -> FROM Users AS u
    -> LEFT OUTER JOIN
    -> q2 ON u.id = q2.userid;

SELECT * FROM q2l;
+----+-----------------+
| id | questions2      |
+----+-----------------+
|  1 | 'How are you?'  |
|  2 | 'Good and you?' |
+----+-----------------+

子查询 7：

CREATE VIEW vt AS
    -> SELECT u.name, 
    -> ql.questions, 
    -> q1l.questions1, 
    -> q2l.questions2
    -> FROM 
    -> Users AS u
    -> INNER JOIN
    -> ql ON u.id = ql.id
    -> INNER JOIN
    -> q1l ON ql.id = q1l.id
    -> INNER JOIN
    -> q2l ON q1l.id = q2l.id;
    
SELECT * FROM vt;
+------+----------------------------------------+------------------------+-----------------+
| name | questions                              | questions1             | questions2      |
+------+----------------------------------------+------------------------+-----------------+
| Jim  | 'How are you?', 'Whats going on here?' | 'Whats going on here?' | 'How are you?'  |
| Joe  | 'Good and you?'                        | NULL                   | 'Good and you?' |
+------+----------------------------------------+------------------------+-----------------+

最终查询：

CREATE VIEW v AS
    -> SELECT name AS username,
    -> COALESCE(questions, "") AS questions,
    -> COALESCE(questions1, "") AS "Questions Page 1",
    -> COALESCE(questions2, "") AS "Questions Page 2"
    -> FROM vt;
    
SELECT * FROM v;
+----------+----------------------------------------+------------------------+------------------+
| username | questions                              | Questions Page 1       | Questions Page 2 |
+----------+----------------------------------------+------------------------+------------------+
| Jim      | 'How are you?', 'Whats going on here?' | 'Whats going on here?' | 'How are you?'   |
| Joe      | 'Good and you?'                        |                        | 'Good and you?'  |
+----------+----------------------------------------+------------------------+------------------+