Translation of a (Normal Form) ER Diagram to a RDB. ❑ A Normal Form for ER Question: Is the resulting relational schema in normal form? Advantages. Database Questions and Answers – Entity-Relationship Diagrams This set of Database Multiple Choice Questions & Answers (MCQs). The entity relationship (ER) data model has existed for over 35 years. If we refer back to our COMPANY database, examples of an independent entity include.
Second is logical design. More detailed than a conceptual ERD, a logical ERD includes more information about the data entities and their relationships. However, a logical design ERD is independent of any technology on which its design could be implemented.
Last is physical design, which depicts the elements and characteristics of a database relative to the technology on which it is to be implemented. This typically includes the specification of entity keys, attribute characteristics, and finite relationship and foreign key definitions.
Entities and Tables In an ERD, entities are represented as tables that typically include only the attribute names at the conceptual level. In this ERD, for example, each entity - such as students, classes, buildings, and books - is defined as a table, and each entry - such as a student, a class, or a book - occupies a row in the table. An entity definition in an ERD Also, each of the attributes, or fields, of the entity creates a column in the table, and all of the rows have the same attributes.
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.
- The ER Diagram
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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.
What is the Entity-Relationship Model? - Diagram & Examples
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. A passenger has given names, a surname, and a unique email address.
What is the Entity-Relationship Model? - Diagram & Examples - Video & Lesson Transcript | pdl-inc.info
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: N relationships between it and the Passenger and Flight entities.
Identifying such entities allows us to get a better picture of the requirements.
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There are no requirements to capture passenger details such as age, gender, or frequent-flier number. 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.
Data Modeling and Entity Relationship Diagram (ERD)
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.