Chapter 4 - Entity Relationship (ER) Modeling

mandatory participation

A relationship in which one entity occurrence must have a corresponding occurrence in another entity. For example, an EMPLOYEE works in a DIVISION.

identifying relationship

A relationship in which related entities are existence-dependent. Also called a strong relationship or strong identifying relationship because the dependent entity's primary key contains the primary key of the parent entity.

non-identifying relationship

A relationship in which the primary key of the dependent entity does not contain the primary key of the related parent entity.

weak relationship

A relationship in which the primary key of the related entity does not contain a primary key component of the parent entity. Also known as a non-identifying relationship.

strong (identifying) relationship

A relationship that occurs when two entities are existence-dependent; from a database design perspective, this relationship exists whenever the primary key of the related entity contains the primary key of the parent entity.

unary relationship

An ER term used to describe an association within an entity. For example, a COURSE might be a prerequisite to another COURSE.

composite attribute

An attribute that can be further subdivided to yield additional attributes. For example, a phone number such as 615-898-2368 may be divided into an area code 615, an exchange number 898, and a four-digit code 2368. Compare to simple attribute.

relational schema

The organization of a relational database as described by the database administrator.

iterative process

A process based on repetition of steps and procedures.

existence-independent

A property of an entity that can exist apart from one or more related entities. Such a table must be created first when referencing an existence-dependent table.

existence-dependent

A property of an entity whose existence depends on one or more other entities. In such an environment, the existence-independent table must be created and loaded first because the existence-dependent key cannot reference a table that does not yet exist.

cardinality

A property that assigns a specific value to connectivity and expresses the range of allowed entity occurrences associated with a single occurrence of the related entity.

recursive relationship

A relationship found within a single entity type. For example, an EMPLOYEE is married to an EMPLOYEE or a PART is a component of another PART.

binary relationship

An ER term for an association between two entities. For example, PROFESSOR teaches COURSE.

participants

An ER term for entities that participate in a relationship. For example, in the relationship PROFESSOR teaches CLASS, the teaches relationship is based on the participants PROFESSOR and CLASS.

ternary relationship

An ER term used to describe an association between three entities. For example, a CONTRIBUTOR contributes money to a FUND from which a RECIPIENT receives money.

multivalued attributes

An attribute that can have many values for a single entity occurrence. For example, an EMP_DEGREE attribute might store the string BBA, MBA, PHD to indicate three different degrees held.

single-valued attribute

An attribute that can have only one value.

simple attribute

An attribute that cannot be subdivided into meaningful components. Compare to composite attribute.

derived attribute

An attribute that does not physically exist within the entity and is derived via an algorithm. For example, the Age attribute might be derived by subtracting the birth date from the current date.

weak entity

An entity that displays existence dependence and inherits the primary key of its parent entity. For example, a DEPENDENT requires the existence of an EMPLOYEE.

strong entity

An entity that is existence-independent, that is, it can exist apart from all of its related entities. Also called a regular entity.

regular entity

An entity that is existence-independent, that is, it can exist apart from all of its related entities. Also called a strong entity.

optional participation

In ER modeling, a condition in which one entity occurrence does not require a corresponding entity occurrence in a particular relationship.

composite identifier

In ER modeling, a key composed of more than one attribute.

optional attribute

In ER modeling, an attribute that does not require a value; therefore, it can be left empty.

required attribute

In ER modeling, an attribute that must have a value. In other words, it cannot be left empty.

identifiers

In an ERM, unique names of each entity instance. In the relational model, such identifiers are mapped to primary keys in tables.

connectivity

The classification of the relationship between entities. Classifications include 1:1, 1:M, and M:N.

relationship degree

The number of entities or participants associated with a relationship. A relationship degree can be unary, binary, ternary, or higher.