想不通 sql 加入

Question

我正在使用 nextcloud 通过 forms 应用程序跟踪数据，表oc_forms_v2_submissions包含以下条目：

SELECT * FROM `oc_forms_v2_submissions` WHERE `form_id` = 3;
+----+---------+--------------------------------------------+------------+
| id | form_id | user_id                                    | timestamp  |
+----+---------+--------------------------------------------+------------+
|  8 |       3 | anon-user-96684f301d22e7be44f07780a9bffe06 | 1663789158 |
|  9 |       3 | anon-user-a1eaa4f939b59e00b403c046410788aa | 1663835954 |
| 10 |       3 | anon-user-440d0dbe9c107492b6ec1a06d98004a8 | 1663942458 |
+----+---------+--------------------------------------------+------------+

第二个表是oc_forms_v2_answers

SELECT * FROM `oc_forms_v2_answers`;
+----+---------------+-------------+-----------------------+
| id | submission_id | question_id | text                  |
+----+---------------+-------------+-----------------------+
| 10 |             8 |           7 | foo                   |
| 11 |             9 |           7 | bar                   |
| 12 |            10 |           7 | foo                   |
+----+---------------+-------------+-----------------------+

所以基本上我需要从表提交中获取所有 id 条目，并将它们与 answer 中的 submit_id 匹配，我想从文本列中获取数据。

SELECT oc_forms_v2_submissions.id as submission_id
FROM `oc_forms_v2_submissions` 
RIGHT  JOIN `oc_forms_v2_answers` ON submission_id=oc_forms_v2_answers.submission_id;

到目前为止，这就是我所能想到的，但它只返回 submit_id 字段并且所有内容都重复了三倍：-D

+---------------+
| submission_id |
+---------------+
|             8 |
|             8 |
|             8 |
|             9 |
|             9 |
|             9 |
|            10 |
|            10 |
|            10 |
+---------------+

编辑：

更新后的查询仍然没有让我得到来自 oc_forms_answers 的字段：

SELECT oc_forms_v2_submissions.id as submission_id
FROM `oc_forms_v2_submissions`
RIGHT JOIN `oc_forms_v2_answers` ON oc_forms_v2_submissions.id=oc_forms_v2_answers.submission_id where form_id="3";

Answer 1

那是因为您正在比较相同的列，您需要在 ON 子句中，两个表的链接列

您也可以使用别名，以减少打字时间

RIGHT JOIN 也会将所有答案与提交的内容结合起来，但你永远不会有更多的提交作为答案，所以即使没有答案，LEFT JOIN 也会处理所有提交

SELECT oc_forms_v2_submissions.id as submission_id
FROM `oc_forms_v2_submissions` 
LEFTJOIN `oc_forms_v2_answers` ON oc_forms_v2_submissions.id=oc_forms_v2_answers.submission_id;

Answer 2

这应该可以解决问题（只需更新列和表的正确命名）

SELECT s.id as submission_id, a.txt FROM submissions s 
LEFT JOIN answers a 
ON s.id=a.submission_id;

您可以在db-fiddle中查看此内容。 我已使用您的信息来创建数据库，因此缺少 WHERE 子句，但所有 rest 都应该为您提供所需的结果。