[英]MySQL - Selecting multiple columns from multiple tables
我有三张桌子。 其中两个是独立的无关表(学生和科目),第三个(条目)是将它们都与外键(student_id和subject_id)链接的表。
这是所有带有记录的表:
学生们:
+------------+------------+-----------+---------------------+----------------------+
| student_id | first_name | surname | email | reg_date |
+------------+------------+-----------+---------------------+----------------------+
| 1 | Emma | Harvey | emmah@gmail.com | 2012-10-14 11:14:13|
| 2 | Daniel | ALexander | daniela@hotmail.com | 2014-08-19 08:08:23 |
| 3 | Sarah | Bell | sbell@gmail.com | 1998-07-04 13:16:32 |
+------------+------------+-----------+---------------------+----------------------+
学科:
+------------+--------------+------------+----------------+
| subject_id | subject_name | exam_board | level_of_entry |
+------------+--------------+------------+----------------+
| 1 | Art | CCEA | AS |
| 2 | Biology | CCEA | A |
| 3 | Computing | OCR | GCSE |
| 4 | French | CCEA | GCSE |
| 5 | Maths | OCR | AS |
| 6 | Chemistry | CCEA | GCSE |
| 7 | Physics | OCR | AS |
| 8 | RS | CCEA | GCSE |
+------------+--------------+------------+----------------+
条目:
+----------+---------------+---------------+------------+
| entry_id | student_id_fk | subject_id_fk | entry_date |
+----------+---------------+---------------+------------+
| 1 | 1 | 1 | 2012-10-15 |
| 2 | 1 | 4 | 2011-09-21 |
| 3 | 1 | 3 | 2015-08-10 |
| 4 | 2 | 6 | 1992-07-13 |
| 5 | 3 | 7 | 2013-02-12 |
| 6 | 3 | 8 | 2016-01-14 |
+----------+---------------+---------------+------------+
我该如何选择带有考试委员会OCR(使用条目表)的每个学生的电子邮件( 学生表)和subject_name( 主题表)? 因此,基本上是使用一种开销表从多个表中选择多个列。 所需的输出是这样的:
email | subject_name
------------------------------
emmah@gmail.com | Computing
sbell@gmail.com | Physics
(对不起,我不知道如何在问题框中正确地重新创建表)。
我认为它始于
SELECT student.email, subjects.subject_name
FROM students, subjects
但是在那之后,除了使用联接之外,我没有太多想法。
非常感谢帮助!! 所以谢谢!
您走在正确的轨道上。 您需要做的是使用entries
表使用主键和外键连接其他两个表,如下所示:
SELECT student.email, subjects.subject_name
FROM students
INNER JOIN entries ON students.student_id = entries.student_id_fk
INNER JOIN subjects ON subjects.subject_id = entries.subject_id_fk
WHERE subjects.exam_board = 'OCR'
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