ER Modeling

Entity

a person, place, object, event, concept (often corresponds to a row in a table)

Relationships

Link between entities (corresponds to primary key - foreign key quivalencies in related tables)

Attributes

Properties or characteristics of an entity or relationship type

Entity

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

Entity Type

a collection of entities that share common properties or characteristics

Entity instance

a single occurrence of an entity type

Strong entity

- exists independently of other types of entities

- has its own unique identifier

Weak Entity

- dependent on a strong entity (identifying owner)... cannot exist on its own

Weak Entity

Does not have a unique identifier (only a partial identifier)

Weak Entity

Entity box and partial identifier have double lines

Identifying Relationship

Links strong entities to weak entities

Attributs

property of characteristic of an entity or relationship type

Required attribute

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

Optional Attribute

may not have a value for every entity (or relationship) instance with which it is associated

Composite attribute

an attribute that has meaningful component parts (attributes)

Multivalued attribute

may take on more than one value for a givent entity (or relationship) instance

Derived attribute

values can be calculated from related attribute values (not physically stored in the database)

Identifier

an attribute (or combination of attributes) that uniquely identifies individual instances of an entity type

Candidate Identifier

an attribute that could be an identitfier...satisfies the requirements for being an identifier

Associative Entity

combination of relationship and entity

Unary Relationship

what degreee of a relationship if one entity is related to another of the same entity type

Binary Relationship

what degree of a relationship is when entities of two different types related to each other

Ternary Relationship

what degree of a relationship when entities of three different types related to each other

One-to-One

Each entity in the relationship will have exactly one related entity

One-to-Many

An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maxiumum of one related entity

Many-to-Many

Entities on both sides of the relationship can have many related entities on the other side

Cardinality Constraints

the number of instances of one entity that can or must be associated with each instance of another entity

Time stamp

a time value that is associated with a data value, often indicating when some event occured that affected the data value