How to Convert ER Diagram to Relational Database | Learn Databases
To design the conceptual schema for a database application. Overview of Database Enhanced Entity Relationship (EER) Diagrams. (next week). UML The company is organized into departments. Each department has. E-R model is a high-level conceptual model for database design; Example 1. COMPANY . ER schema diagram for the COMPANY database: ER diaram for the. Database — Modeling: Entity Relationship Diagram (ERD) (Part 5) ERD of a company database — Images were taken from Fundamentals.
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.Introduction to ER model
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.
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- Related Diagrams
- An example of the conceptual design of a library information system
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. UML relationships actually, ER relationships as well may either be of association or of aggregation. As examples of the latter we have: Employees have dependents Departments have a location How do we translate this to tables?
We'll get to this next, but note that a 1: N relationship can be modeled as an attribute of one of the entities the entity on the side of the N.
COMPANY DATABASE ( Entity Relationship Diagram)
N relationships must get their own table. ER-to-relational mapping How do we build a database schema from an ER diagram? We use all the leaf attributes; composite attributes are represented by their ungrouped components.
Keys are also declared. Attributes that were earlier pushed into relationships are not yet included.
We pick one of the two -- say S -- and add to S a column that represents the primary key of T, and all the attributes of R. It is better to choose as S the entity that has total or at least closer to total participation in R. For example, the manages relationship between departments and employees is 1: We also add a foreign key constraint to S, on the new attribute, referring to the primary key of T.
One alternative is to merge S and T into a single relationship; this makes sense only if both have total participation in R. This means that S and T each have the same number of records, and each record s in S corresponds to exactly one t in T. An entity type within ER diagram is turned into a table. You may preferably keep the same name for the entity or give it a sensible name but avoid DBMS reserved words as well as avoid the use of special characters.
Each attribute turns into a column attribute in the table. The key attribute of the entity is the primary key of the table which is usually underlined.
It can be composite if required but can never be null. It is highly recommended that every table should start with its primary key attribute conventionally named as TablenameID. Taking the following simple ER diagram: The initial relational schema is expressed in the following format writing the table names with the attributes list inside a parentheses as shown below for Persons personidname, lastname, email Persons and Phones are Tables.
Multi-Valued Attributes A multi-valued attribute is usually represented with a double-line oval. If you have a multi-valued attribute, take the attribute and turn it into a new entity or table of its own.
Then make a 1: N relationship between the new entity and the existing one.