ERD Notations in Data Modeling. Part 2 – Chen Notation
In systems analysis, a many-to-many relationship is a type of cardinality that refers to the relationship between two entities A and B in which A may contain a. 1. Chapter 6: ER – Entity Relationship Diagram. Major components of ER diagram; Practices Mapping cardinality of a relationship. 1 –1; 1 – Many-one and many-many. Yan Huang - ER. 1- many. Yan Huang - ER . The database will contain at least the following concepts: songs, artists, bands, albums, and genres. Typically, this is the case for your shipment to item relation: In the real life, you'd expect that one shipment contains at least 1 item (people.
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.
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.
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A student must enroll in a program. A student takes the courses that are part of her program. 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.
Data Modeling and Entity Relationship Diagram (ERD)
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.
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.
Many-to-many (data model)
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. An airplane can be involved in any number of flights, while each flight uses exactly one airplane, so the Flies relationship between the Airplane and Flight relationships has cardinality 1: N; because a flight cannot exist without an airplane, the Flight entity participates totally in this relationship.
A passenger can book any number of flights, while a flight can be booked by any number of passengers. N Books relationship between the Passenger and Flight relationship, but considering the issue more carefully shows that there is a hidden entity here: We capture this by creating the intermediate entity Booking and 1: Entity Entities are the basic objects of ERDs. These are the tables of your database.
Entity are nouns and the types usually fall into five classes. Concepts, location, roles, events or things. Specific example of an entity is called an instance. Each isntance becomes a record or a row in a table. For example, the student John Smith is a record in a table called students. Relationships Relationships are the associations between the entities.
Verbs often describe relationships between entities. We will use Crow's Foot Symbols to represent the relationships. Three types of relationships are discussed in this lab. If you read or hear cardinality ratios, it also refers to types of relationships.
One to One Relationship 1: Each student fills one seat and one seat is assigned to only one student. Each professor has one office space.
Entity Relationship Modeling Examples - Learning MySQL [Book]
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.