Question

我有一个带有两个父表的数据库，这些表由连接表中的外键引用，我正在试图弄清楚如何从一个父表中获取所有相关行，并给出一个值另一个。 例如，使用以下表格和列：

schoolA_courses:
id 
course_prefix
course_number 

schoolB_courses: 
id
course_prefix 
course_number

schoolB_equivalencies:
equivalency_id
schoolA_id
schoolB_id
FOREIGN KEY (schoolA_id) REFERENCES schoolA_courses (id)
FOREIGN KEY (schoolB_id) REFERENCES schoolB_courses (id)

看起来像这样的行：

schoolA:
1, MATH, 101
2, SOCI, 100
3, HIST, 211

schoolB:
1, MATH, 100
2, MATH, 110
3, ENGL, 210

schoolB_equivalencies:
1, 1, 1 (school A's MATH 101 equivalent to school B's MATH 100)
2, 1, 2 (school A's MATH 101 equivalent to school B's MATH 110)

给定一个schoolA课程前缀和数字（比方说MATH 101），如何构建SELECT查询以查找schoolB_equivalencies表中列为等效的所有schoolB课程前缀/数字对？例如，通过我的MATH 101虚拟课程，我将如何获得

MATH 100
MATH 110

Answer 1

此查询获取有关原始课程的信息：

SELECT *
FROM schoolA
WHERE course_prefix = 'MATH'
  AND course_number = 101;

此查询获取有关联结表中相应行的信息：

SELECT *
FROM schoolB_ueqivalencies
WHERE schoolA_id = (SELECT id
                    FROM schoolA
                    WHERE course_prefix = 'MATH'
                      AND course_number = 101);

此查询获取有关schoolB：

中相应行的信息

SELECT *
FROM schoolB
WHERE id IN (SELECT schoolB_id
             FROM schoolB_ueqivalencies
             WHERE schoolA_id = (SELECT id
                                 FROM schoolA
                                 WHERE course_prefix = 'MATH'
                                   AND course_number = 101));

SELECT使用联结表从另一个父表中获取值，从一个父表中获取值

1 个答案: