在mysql中用逗号分隔值连接三个表

Posted 2023-02-24

【中文标题】在mysql中用逗号分隔值连接三个表

【英文标题】：Join three tables with comma separated values in mysql

【发布时间】：2020-10-02 23:58:39

【问题描述】：

我有如下 3 个表

**tbl_link**
╔═════════════╦═══════════════╦═══════════════╗
║ subject_id  ║ system_csv_id ║class_csv_id   ║
╠═════════════╬═══════════════╬═══════════════╣
║ 1           ║ 4,3           ║ 5,1,3,4       ║
║ 2           ║ 1,3           ║ 2,3           ║
╚═════════════╩═══════════════╩═══════════════╝

**tbl_system**                           **tbl_class**
╔═══════════╦════════════════════╗       ╔═══════════╦════════════════════╗
║ system_id ║ system_desc        ║       ║ class_id  ║ class_desc         ║                
╠═══════════╬════════════════════╣       ╠═══════════╬════════════════════╣
║ 1         ║ ANY                ║       ║ 1         ║ GRADE 8            ║
║ 2         ║ LOCAL SYSTEM       ║       ║ 2         ║ GRADE 9            ║
║ 3         ║ BRITISH SYSTEM     ║       ║ 3         ║ GRADE 10           ║ 
║ 4         ║ AMERICAN SYSTEM    ║       ║ 4         ║ GRADE 11           ║
╚═══════════╩════════════════════╝       ║ 5         ║ GRADE 12           ║
                                         ╚═══════════╩════════════════════╝

我想查询 tbl_link 并关联 'system_csv_id' -> 'system_desc' 和 'class_csv_id, -> 'class_desc' 以达到以下输出。

╔═════════════╦═══════════════════╦═════════════════════════════════════════╗
║ subject_id  ║ system            ║ class                                   ║
╠═════════════╬═══════════════════╬═════════════════════════════════════════╣
║ 1           ║ AMERICAN,BRITISH  ║ GRADE 12, GRADE 8, GRADE 10, GRADE 11   ║
║ 2           ║ ANY, BRITISH      ║ GRADE 9, GRADE 10                       ║
╚═════════════╩═══════════════════╩═════════════════════════════════════════╝

我进行了一些搜索以找到解决方案。我得到的只是这可以使用（CONCAT 或 GROUP_CONCAT）和 ON FIND_IN_SET() 来实现，但由于我的 SQL 知识非常有限，我无法实现所需的输出。

任何帮助将不胜感激。

【问题讨论】：

这是 2 个常见问题解答——摆脱 CSV 并引入 CSV。请在考虑发布之前阅读您的教科书和/或手册和谷歌任何错误消息或您的问题/问题/目标的许多清晰、简洁和精确的措辞，有和没有您的特定字符串/名称和站点：***.com 和标签；阅读许多答案。如果您发布问题，请使用一个短语作为标题。反映你的研究。请参阅How to Ask 和投票箭头鼠标悬停文本。请在代码问题中给出minimal reproducible example--包括剪切、粘贴和可运行示例。

【参考方案1】：

加入表格并使用group_concat()：

select l.subject_id,
  group_concat(distinct s.system_desc order by find_in_set(s.system_id, l.system_csv_id)) system,
  group_concat(distinct c.class_desc order by find_in_set(c.class_id, l.class_csv_id)) class
from tbl_link l
inner join tbl_system s on find_in_set(s.system_id, l.system_csv_id) 
inner join tbl_class c on find_in_set(c.class_id, l.class_csv_id) 
group by l.subject_id

请参阅demo。 结果：

| subject_id | system                         | class                              |
| ---------- | ------------------------------ | ---------------------------------- |
| 1          | AMERICAN SYSTEM,BRITISH SYSTEM | GRADE 12,GRADE 8,GRADE 10,GRADE 11 |
| 2          | ANY,BRITISH SYSTEM             | GRADE 9,GRADE 10                   |