Entity Relationship Modeling Examples - Learning MySQL [Book]
Use the Entity Relationship symbols to model databases. table inheritance relationship, category to child relationship, view, category and dynamic connector . At the end of this Tutorial, we will have produced a Data Model, which is commonly referred as an Entity-Relationship Diagram, or 'ERD' I hope you find this. Essential to database design, entity-relationship (ER) diagrams are known for Book Box, a subscription that delivers hand-picked children's books every 1, 2.
Anything that is useful to know about an entity or a relationship is an attribute. Since the database must be able to find an entity or a relationship, it must be able to identify it with a set of attributes that is unique. This set of attributes is usually called primary key. After all the relations have been mapped, they are usually also revised to include cardinalities; a cardinality specifies the number of entities related in a relationship.
This information is needed when the database is created. Depending on cardinality and the level of database normalisationit may be necessary to introduce additional entities and relationships.
Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
Creating a database When everything has been described, it is easy to create a relational database, as follows: Every type of entity can directly be mapped to a relation "database table" Every type of attribute can directly be mapped to attributes "columns" in the database table Attributes can directly be mapped to attributes "columns" in the database table Limitations An ER model describes the relations between entities. It is adapted to represent relational data. Data that cannot easily be put into this form should not be modelled with an ER model.
The university database captures the details of students, courses, and grades for a university.
The flight database stores an airline timetable of flight routes, times, and the plane types. The next section explains these databases, each with its ER diagram and an explanation of the motivation for its design.
The Music Database The music database stores details of a personal music library, and could be used to manage your MP3, CD, or vinyl collection. It ignores the requirements of many music genres, making it most useful for storing popular music and less useful for storing jazz or classical music.
The collection consists of albums. An album is made by exactly one artist. An artist makes one or more albums. An album contains one or more tracks Artists, albums, and tracks each have a name. Each track is on exactly one album. Each track has a time length, measured in seconds. When a track is played, the date and time the playback began to the nearest second should be recorded; this is used for reporting when a track was last played, as well as the number of times music by an artist, from an album, or a track has been played.
Conversely, each play is associated with one track, a track is on one album, and an album is by one artist. The attributes are straightforward: The track entity has a time attribute to store the duration, and the played entity has a timestamp to store when the track was played.
How to Draw a Database Model Diagram - Simple Tutorial
If you wanted to use the music database in practice, then you might consider adding the following features: 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. 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.
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.
How to Draw a Database Model Diagram
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. 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.
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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.