Objects in a CDM
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The following items are available in a CDM:
See also: Objects in any Diagram.
Domains make it easier to identify the types of information in your model, assigning domains to attributes help you standardize data characteristics for entity attributes in different entities.
Just like an "attribute" of an Entity, each Domain has a datatype. In addition to that, each domain can have a set of allowed values or have a minimum and maximum value defined, specified maximum length or precision and a scale. This makes the possible values in a "domain" a sub-set of the available values for a given datatype. Domains can be used as the "datatype" of an attribute instead of a raw datatype.
A Domain can also have a "default value" in those cases no value is used.
Domains are not visible in the diagram itself, but are visible in the Diagram Explorer.
Each entity represents a real world "object", in the abstract sense of the word. An entity contains data about real world concepts, eg: Book, Invoice, Car, Person. Each entity should have one or more attributes describing the entity, eg: First Name, Last Name, Address (of entity Person). Attributes can be optional (= no value required) or required.
In addition to normal attributes, an entity should have an "identifier" which value uniquely identifies an instance of the entity. An "identifier" can be a single attribute (eg: EmployeeNumber) or a combination of attributes. Each of the identifier attributes will be required to have a value.
Entities can have relationships with other entities, eg: a Person has a Car.
One entity can have something to do with another entity, eg: a Person has a Car. This is called a "relationship" between entities.
A relationship can have many different forms, called "roles". In "a Person has a Car", there are two roles: "has" and "belongs to". Each Car belongs to a Person and each Person can have a car.
Besides roles, there's "cardinality", in our example: "a Person has 1 Car" is different from "a Person can have 1 or more Cars" which is different from "a Person can have zero or more Cars". These different cardinalities are available for each relationship.
Next: Datatypes in a CDM