sql - 设计表格时如何实现一对一、一对多和多对多的关系？

Question

谁能解释一下如何在设计表格时实现一对一、一对多和多对多关系？

Answer 1

一对一:使用引用表的外键:

student: student_id, first_name, last_name, address_id
address: address_id, address, city, zipcode, student_id # you can have a
                                                        # "link back" if you need

您还必须在外键列 ( addess.student_id ) 上设置唯一约束，以防止子表 ( address ) 中的多行与引用表 ( student ) 中的同一行相关。

一对多 :在链接回“一”方的关系的多方使用外键:

teachers: teacher_id, first_name, last_name # the "one" side
classes:  class_id, class_name, teacher_id  # the "many" side

多对多 :使用连接表( example ):

student: student_id, first_name, last_name
classes: class_id, name, teacher_id
student_classes: class_id, student_id     # the junction table

示例查询:

 -- Getting all students for a class:

    SELECT s.student_id, last_name
      FROM student_classes sc 
INNER JOIN students s ON s.student_id = sc.student_id
     WHERE sc.class_id = X

 -- Getting all classes for a student: 

    SELECT c.class_id, name
      FROM student_classes sc 
INNER JOIN classes c ON c.class_id = sc.class_id
     WHERE sc.student_id = Y