问题
I was simply wondering, how an ISA relationship in an ER diagram would translate into tables in a database.
Would there be 3 tables? One for person, one for student, and one for Teacher?
Or would there be 2 tables? One for student, and one for teacher, with each entity having the attributes of person + their own?
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?
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.
回答1:
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.
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).
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).
If the disjointness is different, then it's a different story.
回答2:
there are 4 options you can use to map this into an ER,
option 1
- Person(SIN,Name)
- Student(SIN,GPA)
- Teacher(SIN,Salary)
option 2 Since this is a covering relationship, option 2 is not a good match.
- Student(SIN,Name,GPA)
- Teacher(SIN,Name,Salary)
option 3
- Person(SIN,Name,GPA,Salary,Person_Type) person type can be student/teacher
option 4
- Person(SIN,Name,GPA,Salary,Student,Teacher) Student and Teacher are bool type fields, it can be yes or no,a good option for overlapping
Since the sub classes don't have much attributes, option 3 and option 4 are better to map this into an ER
回答3:
It depends entirely on the nature of the relationships.
IF the relationship between a Person and a Student is 1 to N (one to many), then the correct way would be to create a foreign key relationship, where the Student has a foreign key back to the Person's ID Primary Key Column. Same thing for the Person to Teacher relationship.
However, if the relationship is M to N (many to many), then you would want to create a separate table containing those relationships.
Assuming your ERD uses 1 to N relationships, your table structure ought to look something like this:
CREATE TABLE Person ( sin bigint, name text, PRIMARY KEY (sin) );
CREATE TABLE Student ( GPA float, fk_sin bigint, FOREIGN KEY (fk_sin) REFERENCES Person(sin) );
and follow the same example for the Teacher table. This approach will get you to 3rd Normal Form most of the time.
来源：https://stackoverflow.com/questions/18992653/entity-relationship-diagram-how-does-the-is-a-relationship-translate-into-table