I have these entities in ER model from my project:
Student
Professor
Subject
Should I connect them with ternary relationship, or each one with binary relationships? I used ternary relationship. Maybe i don't have the best possible name for the relationship, but it involves grading.
And also if i should use ternary relationship which Ids from connected tables should i use as primary key for the table which will represent relationship in database
Based on your diagram, it seems that each Student/Subject combination determines a Professor. Let's compare this with binary relationships:
A binary relationship between Student and Subject would allow you to record which Students are taking which Subjects, without specifying which Professor. Is that valid for your system, or is it necessary to capture the Professor for every Student/Subject combination?
A binary relationship between Student and Professor would record a Professor for each Student, or possibly a set of of Professors for each Student. Is this useful without knowing which subject each Professor teaches to the Student?
A binary relationship between Subject and Professor would record one or more Professors for each subject. That could be useful to capture what a Professor is able or qualified to teach, as opposed to what they're assigned or scheduled to teach.
I've worked on two school administration systems, and we captured (Student PK, Subject PK) (subjects taken by students) and (Professor PK, Subject PK) (subjects that can be taught be professors), but instead of a ternary relationship (Student PK, Subject PK, Professor), we defined a subject group or section Sections (SectionID PK, Subject, Professor) and student section allocation (Student PK, Subject PK, SectionID). Subject groups or sections provide a convenient place to attach further attributes, like Room number, timetabling arrangements, tuition language, etc.
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I am learning about ER modelling about Database systems. My problem is that there is a entity called books,enitity named user and I want to create a borrows relationship between user and book with attribute issue date. I modelled it as described but it was pointed that borrows cannot be a relationship because a same user can borrow a book twice. Can anybody explain me what this is as I am using issue data as an attribute so records in borrows relationship would not collide as I will use PK as userid,bookid and issue date. How can I model this accurately? I am a little confused in this.
In the ER model, entity relations consist of attributes of a single entity set, in which the PK identifies only one entity set. Relationship relations have a composite PK that represents two or more entity sets.
Your question uses a composite PK that represents two entity sets (userid and bookid) and a value set (issue date). Strictly speaking, it's neither an entity relation nor a relationship relation. It's a combination of a relationship relation (two entity keys) and a weak entity set (issue date functions similar to a weak key). If we want to be creative, we might call it a weak relationship.
If I was forced to draw an ER diagram for this, I might present it like this:
The ER model isn't a complete logical model (unlike the relational model) and there are some situations which aren't handled well or at all. This is one of those situations.
As per description, User and Book are the entities.
One user can borrow an instance of book.
Similary, one user can borrow multiple instances of book, whether It can be same
instance or various instances.
So every transaction between the User and Book has the Issue Date.
Neither the user nor the book has the Issue Date.
Here, the relationship between User and Book are Many to Many.
The Bridge table is Transaction. We can name it as Borrow also as per your interest.
Now, The user has one to many transactions.
Every Book has one to many transactions.
Every transaction is a combination of a User and Book.
Note: Since every user can have the same book multiple times and at the same day. So we can have a composite primary key of user_id, book_id and Issue_timestamp as there is a chance of redundancy in the Issue Date in the same combination.
I'm having a scenario to create an ER diagram.
Scenario
There are Several Regions. One region can have several Business Chains. Each Business chain have one region to cover. One Business chain have several outlets. Customers can use this system to connect to any Business chain. Employees of a Business chain can be assigned to any outlet by the admin of the particular Business chain................
My Question is how am I suppose to handle user details and login details in the above ER diagram (or in the application)
Should I use two separate entities as "Customer" and "Employee"???
Should I use one entity as "User"? If so how to handle the above case of handling emplyees' roster
I suggest you combine customers, employees and users into persons, and use subtyping for each of the roles in which a person may occur:
I left out any indication of overlapping/disjoint subsets, you can fill them in based on your requirements. Implementing disjoint subsets would require adding some additional type indicators and check constraints to the tables below.
Physically, the diagram above would translate into a set of tables like:
person (person_id PK, first_name, last_name, ...)
user (person_id PK/FK, username, password_hash, ...)
customer (person_id PK/FK, credit_limit, ...)
employee (person_id PK/FK, salary, ...)
This allows any person with a user record to log in, and you can easily find out whether they're customers, employees (or both) by joining with those tables. You can create customer or employee-specific relationships easily, e.g.
outlet_employees (outled_id PK/FK, employee_person_id PK/FK)
where employee_person_id has an FK constraint referencing person_id in the employee table. You can also make user-specific relationships, or general person relationships, as your requirements dictate
In some ER diagrams there are attributes on the relation.In what occasions should we use attributes on the relation
Attributes on relationships allow you to record facts about the relationship as opposed to one of the entities that make up the relationship. Some examples:
A marriage between two people has a date and venue
A student's class allocation may have an assigned seat
Popularity ratings by customers on products
Here's an ER example of an attribute on a relationship between student and class:
While the semantics of ER seem familiar and aid its popularity, logically the distinction between entities and relationships is artificial and unnecessary. Entities can have composite keys, and unary relations aren't unusual from an n-ary relational point of view.
So I have this small problem, I am trying to make an ER Diagram and I have a student. For the student entity it says:
Record which school a student is in. A student must be attending one school. All schools contain at least one student.
You are to record the following information schools: their names(must be unique), # of students attending, and the principal name.
When I read this, it seems like "school" is a composite attribute or an entirely separate entity. Since it says "A student must be attending one school. All schools contain at least one student" should I make it an entity because I can't think of how to show this relationship via a composite attribute.
As I see it, school is a separate entity. A composite attribute would be something like a 'name' which consists of a 'first name' and a 'last name'. The school only consists of school and student only consists of student in your example.
I'm a bit confused on what a many to many relationship is. I'm wondering if the following is a many to many relationship:
A student at a school has many clubs. A club at a school has many students. Let's say that the student has many attributes: firstname, lastname, phone, age, email, etc. A club only has one attribute: a name.
When I make a new club, I want to be able to give the club a name and one or more students. Upon making the club, I want that club to be associated with those students and those students to be associated with that club.
When I make a new student, I want to be able to give the student a firstname, last name, etc, and one or more clubs. Upon making the student, I want that student to be associated with those clubs and those clubs to be associated with that student.
I also want to display a club's students and a student's clubs on their show pages.
I've read that a many to many relationship is when you have a join table that lets you access common attributes of the resulting students and clubs, but there are no common attributes in my case.
Do I have a many to many relationship here? If so, do I use a HABTM or has_many, through relationship?
Actually yes you DO have common attributes.
You stated yourself that a Student has many Clubs
And a Club has many Students.
What is in common? Students and Clubs.
What now follows is to define what a Student and a Club actually are, which you already did.
A Student is a combination of firstname, last name, etc... What you have not specified is what makes a Student UNIQUE. A club also must be defined as to what will make it UNIQUE. While for academic purposes, you could say the name is what makes it unique, in real live, that would probably not be the best solution.
Usually for performance purposes, each student is given a unique Autoincrement ID (which is a number).
Same thing can be done with the Club.
You create a 3rd table which is what creates the Many to Many relation.
In that 3rd table, you have 2 columns. One with the Unique Index for the Student, and the other column with the Unique Index for the Club. You simply add an entry on that table in which you wish to relate a student to a club.
Since you can have many students assigned to the same club, and you can have many clubs assigned to the same student, you have a many to many relation.
Edit: As mentioned in another answer, your 3rd table should also declare the combined indexes as unique, so that you don't add the same entry multiple times.
You have a many to many
Create an id for each table that is unique for that table typically an auto incrementing int.
Then a third table that is a junction/intersect table call it X.
Put a row in X with the student id and club id if the student has the club and vice versa. It would have a unique composite key in table X across both id's in it.
The composite would guarantee no duplicate rows in X
Yes indeed there is a many-to-many relationship here, use HABTM. Also, why do you say that there are no attributes in common? Club names and student names are definitely common attributes in this case.