database management ch 4 the relational model of data

The entities that participate in a relationship are

also known as participants,

A composite attribute, not to be confused with a composite key, is

an attribute that can be further subdivided to yield additional attributes. For example, the attribute ADDRESS can be subdivided into street, city, state, and zip code.

A single-valued attribute is

an attribute that can have only a single value. For example, a person can have only one Social Security number, and a manufactured part can have only one serial number. Keep in mind that a single-valued attribute is not necessarily a simple attribute. For instance, a part's serial number (such as SE-08-02-189935) is single-valued, but it is a composite attribute because it can be subdivided into the region in which the part was produced (SE), the plant within that region (08), the shift within the plant (02), and the part number (189935).

An optional attribute is

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

A required attribute is

an attribute that must have a value; in other words, it cannot be left empty. bold in crows foot notationThe ERM uses identifiers—one or more attributes that uniquely identify each entity instance.

a derived attribute is

an attribute whose value is calculated (derived) from other attributes. (doesn't have to be stored in database) dashed line in Chen model, no difference in crows foot

Multivalued attributes are

attributes that can have many values. For instance, a person may have several college degrees, and a household may have several different phones, each with its own number.

If an entity can exist apart from all of its related entities, then it is

existence-independent, and it is referred to as a strong entity or regular entity.

Cardinality

expresses the minimum and maximum number of entity occurrences associated with one occurrence of the related entity. In the ERD, cardinality is indicated by placing the appropriate numbers beside the entities, using the format (x,y). The first value represents the minimum number of associated entities, while the second value represents the maximum number of associated entities.

The concept of relationship strength is based on

how the primary key of a related entity is defined.

An entity is said to be existence-dependent if

it can exist in the database only when it is associated with another related entity occurrence. In implementation terms, an entity is existence-dependent if it has a mandatory foreign key—that is, a foreign key attribute that cannot be null.

The ERM uses identifiers—

one or more attributes that uniquely identify each entity instance.

Key attributes are also underlined in a frequently used shorthand notation for the table structure, called a relational schema, that uses the following format:

see pic.

Optional participation means

that one entity occurrence does not require a corresponding entity occurrence in a particular relationship.] small o in crows foot notation

Mandatory participation means

that one entity occurrence requires a corresponding entity occurrence in a particular relationship.

A relationship degree indicates

the number of entities or participants associated with a relationship.

A strong (identifying) relationship exists when

the primary key of the related entity contains a primary key component of the parent entity. solid line in crows foot

A weak relationship, also known as a non-identifying relationship, exists if

the primary key of the related entity does not contain a primary key component of the parent entity. dashed line in crows foot

he term connectivity is used to describe

the relationship classification.

relations ship names are active or passive

verbs

A unary relationship exists

when an association is maintained within a single entity. A binary relationship exists when two entities are associated. A ternary relationship exists when three entities are associated.

Cardinality denotes how many instances of one object are related to instances of another object.

Cardinality can be shown in crow's feet notations, utilizing a straight line for 1 and a crow's foot for many.

Modality denotes an instance of a specific entity is optional or mandatory in a relationship.

Modality is expressed with a straight line for modality 1 or a circle for modality 0.

a weak entity. is one that meets two conditions:

The entity is existence-dependent; it cannot exist without the entity with which it has a relationship. The entity has a primary key that is partially or totally derived from the parent entity in the relationship.

2 options

Within the original entity, create several new attributes, one for each component of the original multivalued attribute. Create a new entity composed of the original multivalued attribute's components.

Ideally, an entity identifier is composed of only a single attribute. However, it is possible to use a composite identifier,

a primary key composed of more than one attribute.