实体关系图。 IS A关系如何转换为表？ [英] Entity Relationship Diagram. How does the IS A relationship translate into tables?

问题描述

我只是想知道，如何在ER图中的ISA关系将转换为数据库中的表。

I was simply wondering, how an ISA relationship in an ER diagram would translate into tables in a database.

有3张桌子吗？一个人，一个为学生，一个为老师？

Would there be 3 tables? One for person, one for student, and one for Teacher?

还是会有两张桌子？一个用于学生，一个用于教师，每个实体具有个人的属性和他们自己的属性？

Or would there be 2 tables? One for student, and one for teacher, with each entity having the attributes of person + their own?

或者会有一个表包含所有4个属性，表中的正方形是空的，取决于它是否是学生或老师在行中？

Or would there be one table with all 4 attributes and some of the squares in the table being null depending on whether it was a student or teacher in the row?

注意：我忘记添加这一点，但有完全覆盖的ISA

NOTE: I forgot to add this, but there is full coverage for the ISA relationship, so a person must be either a studen or a teacher.

推荐答案

假设这种关系是强制性的（正如你所说， 是一名学生或教师）和不相交（一个人既是学生也是教师，但不是两者），最好的解决方案是有两张表，一张给学生，一张给老师。

Assuming the relationship is mandatory (as you said, a person has to be a student or a teacher) and disjoint (a person is either a student or a teacher, but not both), the best solution is with 2 tables, one for students and one for teachers.

如果参与是可选的（这不是你的情况，但让我们把它作为完整性），那么3表选项是要走的路， Person（PersonID，Name）表，然后是另外两个引用Person表的表，例如
Student（PersonID，GPA），PersonID为PK和FK引用Person（PersonID）。

If the participation is instead optional (which is not your case, but let's put it for completeness), then the 3 tables option is the way to go, with a Person(PersonID, Name) table and then the two other tables which will reference the Person table, e.g. Student(PersonID, GPA), with PersonID being PK and FK referencing Person(PersonID).

1表选项可能不是最好的方法，并且它将生成几个具有空值的记录（如果某人是学生，则只有教师的属性将为空，反之亦然）。

The 1 table option is probably not the best way here, and it will produce several records with null values (if a person is a student, the teacher-only attributes will be null and vice-versa).

如果不相交是不同的，那就是另一回事了。

If the disjointness is different, then it's a different story.

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