Some Definitions

As discussed earlier, the an initial step in the database design procedure is to identify the organization data requirements. As soon as the data requirements have actually been identified, the next step is to design the data together a theoretical schema for the database. The Entity Relationship design (ERM) was an initial described by Chen in 1976 and also is widely welcomed as the conventional for conceptual data modelling in the relational database environment. The ERM is a detailed, logical representation of the entities, associations and data facets for one organisation. Because theoretical models space only concerned with what is stood for in the database, the ERM does no include materials to represent just how the version is implemented.

You are watching: A _____ attribute is one that cannot be subdivided.

The Entity partnership Diagram (ERD) is a graphical representation of an ERM and also is currently one the the most popular and also widely provided database design tools.

Components that the ER version

The 3 main materials of the ER model are entities, attributes and relationships.

In ERM terms, an entity is a "thing" within the organisation, the we want to keep information about, such as a customer, employee or course. In other words, an entity in an ERM actually describes a table, and rows within the table are referred to as reality occurrences.

Entities are represented by rectangles comprise the surname of the entity. Entity names must be singular and also in resources letters.

Each reality has characteristics which space the properties of every entity. Characteristics will be implemented as columns in the tables. Every attribute has a domain which mentions the set of feasible values an attribute can have. Because that instance, the selection of worths for a telephone extension may be mentioned as a collection of creature numbers in between 4000 and 4999. An characteristics domain is not displayed in ER diagrams, however is tape-recorded in the data dictionary.

Attributes are stood for by ovals with lines connecting them to the entity.

Attributes deserve to be of miscellaneous types. A composite attribute have the right to be subdivided right into smaller parts. Because that example, one attribute Name have the right to be subdivided into an initial Name and also Last Name. Qualities that can not be subdivided are called basic attributes. First Name and also Last Name are now basic attributes.

Most features have only a single value and as such are called solitary valued attributes. For example, a Teacher deserve to have just one Last name or a Subject deserve to have only one subject Code. Multivalued attributes can have an ext than one value. For example, a Student could have more than one Certificate or a room may have several Extensions.

There space a number of diagramming conventions offered to represent multivalued attributes. Some conventions implement the multivalued attribute together an oval through a twin line approximately it. Others use a dual bar to affix the attribute to the entity. Usage either an approach in your own diagramming.

A key attribute is an attribute that has a unique value because that each entity occurrence. In various other words, a key attribute is provided to determine each row uniquely. Because that example, a subject Code will uniquely recognize each topic as no 2 subjects can have the very same Subject Code.

Key features are represented by underlining the name.

A partnership is the association in between entities or entity occurrences. For example, a Teacher teaches topics or a topic has Offerings. You will certainly look at relationships in an ext detail in the following section .

There are several actions in the E-R verification process. These room outlined in another section.

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Relationships are represented by diamonds with straight lines connecting the participating entities.

Diagrams for ER contents

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