Chapter 2: Modeling: Entities and attributes

Required attribute

- an attribute that must have a value for every entity(or relationship) instance with which it is associated

Strong entity type

- an entity that exist independently of other entity types

Guidelines for defining entity types

1. An entity type definition usually starts with "..." 2. An entity type definition should include a statement of what the unique characteristic is for each instance of the entity type 3. An entity type definition should make clear what instances are included and not included 4. An entity type definition often includes a description of when an instance of the entity type is created and deleted 5. For some entity the definition must specify when an instance might change into an instance of another entity type 6. For some entity the definition must specify what history is to be kept about instances of the entity type

Criteria for selecting identifiers

1. Not change its value over the life of each instance of the entity type 2. Each instance of the entity, the attribute is guaranteed to have valid values and not be null or unknown 3. Avoid intelligent identifiers, classifications and locations 4. Consider substituting single-attribute surrogate identifiers for large composite identifiers

Guidelines for naming entity types

1. Singular noun 2. An entity type name should be specific to the organization 3. An entity type should be concise, using as few words as possible 4. An abbreviation or a short name 5. Event entity types 6. Name

Guidelines for naming attributes

1. Singular noun phrase 2. Be unique 3. Each attribute name should follow a standard format 4. Entity type name is the name of the entity with which the attribute is associated 5. Class is a phrase from a list of phrases defined by the organization that are the permissible characteristics or properties of entities 6. Qualifier- is a phrase from a list of phrase defined by the organization that are used to place constraints on classes 7. Similar attributes- of different entity types should use the same qualifiers and classes as long as those are the names used in the organization. 8. Domains- same possible set of values

Guidelines for Defining attributes

1. What the attribute is and possibly why it is important 2. What is included and not included 3. Aliases, alternative names 4. The source of values for the attribute 5. An attribute definition (or other specification is a CASE tools repository) also should indicate if a value for the attribute is required or optional 6. Required means that an entity instance must always have a value for this attribute, not just when an instance is created 7. Attribute definition may also indicate whether a value for the attribute may change once a value is provided and before the entity instance is deleted 8. For multi-valued attribute, the attribute definition should indicate the maximum and minimum number of occurrences of an attribute value for an entity instance 9. An attribute definition may also indicate any relationships that attribute has with other attributes

Simple (or atomic) attribute

An attribute that cannot be broken down into smaller components that are meaningful to the organization

Composite attribute

An attribute that has meaningful component parts (attributes)

Entity type versus system input, output or user

Easy to confuse data entities with other elements of an overall information systems model, a true data entity will have many possible instances, each with a distinguished characteristic, as well as one or more other descriptive pieces of data

Identifying owner

The entity type on which the weak entity type depends

Entity type

a collection of entities that share common properties or characteristics with metadata

Entity

a person, a place, an object, an event, or concept in the user environment about which the organization wishes to maintain data

Attributes

a property or characteristic of an entity or relationship type that is of interest to the organization. With each attribute word's first letter capitalized

Entity instance

a single occurrence of an entity type

Identifier attribute

an attribute (or combination of attributes) whose value distinguishes instances of entity type. (Not a person's name)

Optional attribute

an attribute that may not have a value for every entire (or relationship) instance with which it is associated

Multivalued attribute

an attribute that may take on more than one value for a given entity (or relationship) instance

Derived attribute

an attribute whose values can be calculated from related attribute values

Weak entity type

an entity type whose existence depends on some other entity type

Composite Identifier

an identifier that consists of a composite attribute

Identifying relationship

the relationship between a weak entity type and its owner