Entity–relationship model - Wikipedia
Database design: ER diagrams are used to model and design relational databases, in terms of logic and business rules (in a logical data model) and in terms of. This definition explains the meaning of entity relationship diagram, also known as ERD, and how the data model can be used to design relational databases. The ER model defines the conceptual view of a database. It works around real- world entities and the associations among them. At view level, the ER model is.
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.
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.
Entity Relationship Modeling Examples - Learning MySQL [Book]
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. Entity are nouns and the types usually fall into five classes. Concepts, location, roles, events or things.
Learning MySQL by Hugh E. Williams, Saied M.M. Tahaghoghi
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.Introduction to ER model
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.
ER Model Basic Concepts
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. The first is the 'fan trap'. It occurs with a master table that links to multiple tables in a one-to-many relationship. The issue derives its name from the way the model looks when it's drawn in an entity—relationship diagram: This type of model looks similar to a star schemaa type of model used in data warehouses.
When trying to calculate sums over aggregates using standard SQL over the master table, unexpected and incorrect results. The solution is to either adjust the model or the SQL.
- Data Modeling
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This issue occurs mostly in databases for decision support systems, and software that queries such systems sometimes includes specific methods for handling this issue. The second issue is a 'chasm trap'.
A chasm trap occurs when a model suggests the existence of a relationship between entity types, but the pathway does not exist between certain entity occurrences.
For example, a Building has one-or-more Rooms, that hold zero-or-more Computers. One would expect to be able to query the model to see all the Computers in the Building. However, Computers not currently assigned to a Room because they are under repair or somewhere else are not shown on the list. Another relation between Building and Computers is needed to capture all the computers in the building. This last modelling issue is the result of a failure to capture all the relationships that exist in the real world in the model.
See Entity-Relationship Modelling 2 for details.