显示多条记录的多重连接

Question

我有两个表 - 类似于下面。第一个表存储有关每个作业的信息：

+--------+------------+
| JOB_ID |  JOB_NAME  |
+--------+------------+
|        |            |
| 1      | Test Job 1 |
|        |            |
| 2      | Test Job 2 |
|        |            |
| 3      | Test Job 3 |
|        |            |
| 4      | Test Job 4 |
|        |            |
| 5      | Test Job 5 |
|        |            |
| 6      | Test Job 6 |
|        |            |
| 7      | Test Job 7 |
+--------+------------+

第二个表，存储与每个工作相关的一个或多个“主题”：

+------------+--------+------------------+
| SUBJECT_ID | JOB_ID |   SUBJECT_NAME   |
+------------+--------+------------------+
|            |        |                  |
| 1          | 1      | Test Subject #1  |
|            |        |                  |
| 2          | 1      | Test Subject #2  |
|            |        |                  |
| 3          | 1      | Test Subject #3  |
|            |        |                  |
| 4          | 2      | Test Subject #4  |
|            |        |                  |
| 5          | 3      | Test Subject #5  |
|            |        |                  |
| 6          | 3      | Test Subject #6  |
|            |        |                  |
| 7          | 3      | Test Subject #7  |
|            |        |                  |
| 8          | 4      | Test Subject #8  |
|            |        |                  |
| 9          | 5      | Test Subject #9  |
|            |        |                  |
| 10         | 6      | Test Subject #10 |
|            |        |                  |
| 11         | 7      | Test Subject #11 |
|            |        |                  |
| 12         | 7      | Test Subject #12 |
|            |        |                  |
| 13         | 7      | Test Subject #13 |
+------------+--------+------------------+

目前，我有一条 SQL 语句，显示所有工作，以及两个字段中的前两个“主题”，结果如下：

+--------+------------+------------------+------------------+
| JOB_ID |  JOB_NAME  |    SUBJECT 1     |    SUBJECT 2     |
+--------+------------+------------------+------------------+
|        |            |                  |                  |
| 1      | Test Job 1 | Test Subject #1  | Test Subject #2  |
|        |            |                  |                  |
| 2      | Test Job 2 | Test Subject #4  |                  |
|        |            |                  |                  |
| 3      | Test Job 3 | Test Subject #5  | Test Subject #6  |
|        |            |                  |                  |
| 4      | Test Job 4 | Test Subject #8  |                  |
|        |            |                  |                  |
| 5      | Test Job 5 | Test Subject #9  |                  |
|        |            |                  |                  |
| 6      | Test Job 6 | Test Subject #10 |                  |
|        |            |                  |                  |
| 7      | Test Job 7 | Test Subject #11 | Test Subject #12 |
+--------+------------+------------------+------------------+

这是由以下 SQL 产生的：

SELECT   job.job_id as 'Id',  
job.job_name as 'JobName', 
Subjects1.Subject as 'Subject 1', 
Subjects2.Subject as 'Subject 2', 
LEFT JOIN job_Subjects as Subjects1 ON Subjects1.job_id = job.job_id AND Subjects1.Subject_id = (SELECT Subject_id FROM job_Subjects WHERE job_Subjects.job_id = job.job_id ORDER BY job_Subjects.Subject_index LIMIT 1) 
LEFT JOIN job_Subjects as Subjects2 ON Subjects2.job_id = job.job_id AND Subjects2.Subject_id = (SELECT Subject_id FROM job_Subjects WHERE job_Subjects.job_id = job.job_id ORDER BY job_Subjects.Subject_index LIMIT 1,1) 

但是，我还需要搜索主题的名称（job_subjects.subject 字段）。

我尝试了以下方法：

• 添加了“WHERE job_subjects.subject LIKE '%Test Job 3%'” - 但是我收到以下错误：“'where 子句'中的未知列 'job_subjects.subject'”
• 添加了另一个连接“LEFT JOIN job_subjects as SearchSubjects ON SearchSubjects .job_id = job.job_id”，然后添加了“WHERE SearchSubjects.subject LIKE '%Test Job 3%'”但是我似乎得到了多个 JOB 结果（其中，它似乎会为其找到的每个主题结果生成一个作业行结果）。

如何搜索主题表，同时保持两个连接的原样？

Answer 1

MySQL 有一个很好的扩展，你可以使用HAVING 子句来做你想做的事。首先，我建议您对列名使用反引号 - 或使用不需要转义的名称。

SELECT job.job_id as Id, job.job_name as JobName, 
       Subjects1.Subject as Subject1, 
       Subjects2.Subject as Subject2
FROM ?? LEFT JOIN
     job_Subjects Subjects1
     ON Subjects1.job_id = job.job_id AND
        Subjects1.Subject_id = (SELECT Subject_id FROM job_Subjects WHERE job_Subjects.job_id = job.job_id ORDER BY job_Subjects.Subject_index LIMIT 1
                               ) LEFT JOIN
     job_Subjects Subjects2
     ON Subjects2.job_id = job.job_id AND
        Subjects2.Subject_id = (SELECT Subject_id FROM job_Subjects WHERE job_Subjects.job_id = job.job_id ORDER BY job_Subjects.Subject_index LIMIT 1,1
                               ) 
HAVING Subject1 LIKE '%Test Job 3%';

您也可以使用子查询来执行此操作。那将是更传统的方法。但是，MySQL 会实现子查询，从而导致一些性能开销。