如何在 MySQL 的多对多关系中以逗号分隔的列表中的一个字段连接数据？

Posted 2023-02-24

【中文标题】如何在 MySQL 的多对多关系中以逗号分隔的列表中的一个字段连接数据？

【英文标题】：How to concatenate data from one field, in a comma-delimited list, in a many-to-many relationship in MySQL?

【发布时间】：2011-10-12 04:47:06

【问题描述】：

我在人员和部门之间存在多对多关系，因为一个人可以在多个部门。

People          Departments
------          -----------
pID  pName      deptID   deptName
1    James      1        Engineering
2    Mary       2        Research
3    Paul       3        Marketing
                4        Communications

People_Departments
------------------
pID   deptID
1     1
1     2
2     2
2     4
3     1
3     2
3     3

我想要的是这个：

pName  deptName
James  Engineering, Research
Mary   Research, Communication
Paul   Engineering, Research, Marketing

如果我使用下面的 SQL 对表执行简单的 LEFT JOIN，我将获得与一个人相关的几行：

SELECT people.pName,
       departments.deptName
FROM people
LEFT JOIN people_departments ON people.pID=people_departments.pID
LEFT JOIN departments ON people_departments.deptID=departments.deptID

我尝试了GROUP_CONCAT 的各种组合，但没有运气。

有什么想法可以分享吗？

【参考方案1】：

    SELECT people.pName,
           GROUP_CONCAT(departments.deptName SEPARATOR ', ') deptName
      FROM people
 LEFT JOIN people_departments 
        ON people.pID = people_departments.pID
 LEFT JOIN departments 
        ON people_departments.deptID = departments.deptID
  GROUP BY people.pID

输出：

+-------+----------------------------------+
| pName | deptName                         |
+-------+----------------------------------+
| James | Engineering, Research            |
| Mary  | Research, Communications         |
| Paul  | Engineering, Research, Marketing |
+-------+----------------------------------+
3 rows in set (0.00 sec)

【讨论】：

实际上，我的组合都不起作用，因为我省略了非常重要的 GROUP BY。我在实际项目中从几个多对多相关的表中获取数据，并在您的帮助下设法获得了我想要的数据。再次感谢！

请注意，在此查询中，people_departments 的 LEFT JOIN 将被转换为 INNER JOIN，因为在 INNER JOIN 的 ON 子句中使用了people_departments.deptID。如果你想列出没有部门的人，两个 JOIN 都应该是 LEFT JOIN。

@PaulSpiegel 很好。更新了查询。

【参考方案2】：

我的解决办法是：

SELECT people.pName,
GROUP_CONCAT(tmp.deptName SEPARATOR ', ') deptName
FROM people 
LEFT JOIN (SELECT people_departments.pID, departments.deptName FROM people_departments LEFT JOIN departments  ON people_departments.deptID = departments.deptID) as tmp
ON tmp.pID = people.pID
GROUP BY people.pID

结果：

+-------+----------------------------------+
| pName | deptName                         |
+-------+----------------------------------+
| James | Engineering, Research            |
| Mary  | Research, Communications         |
| Paul  | Engineering, Research, Marketing |
+-------+----------------------------------+