What is Entity Relationship Diagram (ERD)?
In an entity relationship diagram (ERD), an entity type is represented by a back to our COMPANY database, examples of an independent entity include the. Entity Relationship Modeling Examples Earlier in this chapter, we showed you how to design a database and understand an Entity Relationship (ER) diagram. The ultimate guide to ER diagram including origins, uses, examples, components , limitations and guidelines on how to draw them with Lucidchart.
Support for compilations or various-artists albums, where each track may be by a different artist and may then have its own associated album-like details such as a recording date and time. Under this model, the album would be a strong entity, with many-to-many relationships between artists and albums.
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Playlists, a user-controlled collection of tracks. For example, you might create a playlist of your favorite tracks from an artist. Track ratings, to record your opinion on how good a track is. Source details, such as when you bought an album, what media it came on, how much you paid, and so on.
Album details, such as when and where it was recorded, the producer and label, the band members or sidemen who played on the album, and even its artwork. Smarter track management, such as modeling that allows the same track to appear on many albums. The University Database The university database stores details about university students, courses, the semester a student took a particular course and his mark and grade if he completed itand what degree program each student is enrolled in.Entities, Attributes, and Relationships
We explain the requirements next and discuss their shortcomings at the end of this section. Consider the following requirements list: The university offers one or more programs.
A program is made up of one or more courses. A student must enroll in a program. A student takes the courses that are part of her program.
Data Modeling and Entity Relationship Diagram (ERD)
A program has a name, a program identifier, the total credit points required to graduate, and the year it commenced. A course has a name, a course identifier, a credit point value, and the year it commenced.
Students have one or more given names, a surname, a student identifier, a date of birth, and the year they first enrolled. When he finishes the course, a grade such as A or B and a mark such as 60 percent are recorded. Each course in a program is sequenced into a year for example, year 1 and a semester for example, semester 1. Although it is compact, the diagram uses some advanced features, including relationships that have attributes and two many-to-many relationships.
The ER diagram of the university database In our design: Each student must be enrolled in a program, so the Student entity participates totally in the many-to-one EnrollsIn relationship with Program.
A program can exist without having any enrolled students, so it participates partially in this relationship. As a weak entity, Course participates totally in the many-to-one identifying relationship with its owning Program. This relationship has Year and Semester attributes that identify its sequence position.
Student and Course are related through the many-to-many Attempts relationships; a course can exist without a student, and a student can be enrolled without attempting any courses, so the participation is not total.
When a student attempts a course, there are attributes to capture the Year and Semester, and the Mark and Grade. For a real university, many more aspects would need to be captured by the database.
The airline has one or more airplanes. An airplane has a model number, a unique registration number, and the capacity to take one or more passengers.
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An airplane flight has a unique flight number, a departure airport, a destination airport, a departure date and time, and an arrival date and time.
Each flight is carried out by a single airplane. A passenger has given names, a surname, and a unique email address. A passenger can book a seat on a flight.
The ER diagram of the flight database An Airplane is uniquely identified by its RegistrationNumber, so we use this as the primary key. A Flight is uniquely identified by its FlightNumber, so we use the flight number as the primary key. The departure and destination airports are captured in the From and To attributes, and we have separate attributes for the departure and arrival date and time.
Because no two passengers will share an email address, we can use the EmailAddress as the primary key for the Passenger entity. Each student fills one seat and one seat is assigned to only one student. Each professor has one office space. One to Many Relationship 1: M A single entity instance in one entity class parent is related to multiple entity instances in another entity class child For example: One instructor can teach many courses, but one course can only be taught by one instructor.
One instructor may teach many students in one class, but all the students have one instructor for that class. Many to Many Relationship M: M Each entity instance in one entity class is related to multiple entity instances in another entity class; and vice versa. Each student can take many classes, and each class can be taken by many students.
Each consumer can buy many products, and each product can be bought by many consumers. The detailed Crow's Foot Relationship symbols can be found here. Crow's Foot Relationship Symbols Many to many relationships are difficult to represent. We need to decompose a many to many M: M relationship into two one-to-many 1: Attributes Attributes are facts or description of entities.
They are also often nouns and become the columns of the table. For example, for entity student, the attributes can be first name, last name, email, address and phone numbers.
For example, for a student entity, student number is the primary key since no two students have the same student number. We can have only one primary key in a table. It identify uniquely every row and it cannot be null. Typically you take the primary key field from one table and insert it into the other table where it becomes a foreign key it remains a primary key in the original table. We can have more than one foreign key in a table.
This ERD should include the user accounts, posts and re-posts tables. To get you started, you can follow the example given below. User table The User table collect attributes: