An entity-relationship (ER) diagram is a graphical representation of entities and their For example, in a database of employees, each employee name (A) is. The ultimate guide to ER diagram including origins, uses, examples, ER Diagrams are most often used to design or debug relational databases in the fields of Also known as ERDs or ER Models, they use a defined set of symbols such as. Translation of an E/R model into the logical model (DB. Schema) Modeling is about mapping entities and relationships Example Schema with Attributes.
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Entity Relationship Modeling Examples - Learning MySQL [Book]
Get unlimited access to videos, live online training, learning paths, books, tutorials, and more. No credit card required Entity Relationship Modeling Examples Earlier in this chapter, we showed you how to design a database and understand an Entity Relationship ER diagram. This section explains the requirements for our three example databases—music, university, and flight—and shows you their Entity Relationship diagrams: The music database is designed to store details of a music collection, including the albums in the collection, the artists who made them, the tracks on the albums, and when each track was last played.
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.
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.
Entity–relationship model - Wikipedia
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. 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. Different shapes at the ends of these lines represent the relative cardinality of the relationship. Crow's foot notation was used in the consultancy practice CACI.
With this notation, relationships cannot have attributes. Where necessary, relationships are promoted to entities in their own right: Model usability issues[ edit ] You can help by adding to it. February In using a modeled database, users can encounter two well known issues where the returned results mean something other than the results assumed by the query author.
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.Introduction to ER model
The solution is to either adjust the model or the SQL. 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.
entity-relationship diagram (model)
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. Entity—relationships and semantic modeling[ edit ] Semantic model[ edit ] A semantic model is a model of concepts, it is sometimes called a "platform independent model". It is an intensional model.