Database Applications 2

A recursive relationship refers to a?

unary relationship

How do you express a cardinality that has no upper limit?

(1,N), or well I suppose you could have a different lower bound than 1, but you get the idea.

What are some of the conflicting goals of db design?

-Design standards -Processing speed -Information requirements

What two conditions define a weak entity?

1- The entity is existence-dependent; it cannot exist without the entity with which it has a relationship. 2- The entity has a primary key that is partially or totally derived from the parent entity in the relationship.

What 2 options do db designers have when dealing with multi-valued attributes?

1. Within the original entity, create several new attributes, one for each component of the original multivalued attribute. Like break out all the car colors for a single car into the specific areas thet color. 2 Create a new entity composed of the original multivalued attribute's components. This new entity allows the designer to define color for different sections of the car.

What is a recursive relationship?

A recursive relationship is one in which a relationship can exist between occurrences of the same entity set. Employee is married to employee.

In terms of keys what defines a weak (non-identifying) relationship between entities?

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.

What type of relationships are most ternary and higher degree relationships decomposed into?

Binary relationships

How do you designate weak entities in Chen Notation? Crow's foot?

Chen Notation has a double walled entity rectangle. Crow's Foot has you infer this status from the PK, FK designations. A strong (identifying) relationship indicates that the related entity is weak.

Which ER diagram notation is best used for conceptual design?

Chen notation.

How does the Chen Model represent a derived attribute? How about the Crow's Foot Model?

Chen represents derived attributes by use of a dotted line between the entity and the attribute that is derived. Crow's Model DOESN'T REPRESENT derived attributes. Notice in the Chen Model EMP_DOB is a required attribute with a solid line but EMP_AGE is indicated as derived?

Compare how entities and attributes are represented in the Chen's and Crow's Foot models?

Chen's doesn't really care about space, does it? In Chen's attributes are placed in ovals around the entity, which is in a box. In crow's foot the entity box has its name at the top, with attributes below the line. BOLDFACED attributes are REQUIRED in crow's foot. The non-bold attributes are optional attributes.

How do ERD connectivity and cardinality differ?

Connectivity is used to describe the relationship classification (1:1, 1:M) whereas cardinality expresses the minimum and maximum number of entity occurrences associated with one occurrence of the related entity (isn't this just the domain)? The modality is the optionality of the relationship I think. The 1 or O in the Crow's Foot.

How do you demonstrate a weak relationship in Crow's foot notation?

Dotted line.

In implementation terms when is an entity considered to be existence-dependent?

If it has mandatory FKs. In other words it has FKs that cannot be null.

How is the PK represented in the Relational Schema format?

It is underlined. Attributes are in parentheses following the table name.

How is Chen Notation weird about expressing cardinality?

It places it next to the related entity. 1 professor taught 1 to 4 classes in Chen the (1,4) would be next to the professor. In crow's foot and UML the (1,4) would be on the class side.

Are Single-Valued attributes necessarily simple attributes?

NO! There can be elements that can be decomposed from a Single-Valued attribute. Think of a product serial number that codes different pieces of information within it even though it is a single value.

Are composite attributes the same as composite keys?

No! Composite attributes can be further subdivided into simple attributes. Think like elements of an address or phone number. Composite keys have nothing to do with them.

Describe a Multi-valued attribute. Which ERM type can represent Multi-valued attributes?

One that can have many values simultaneously. Like college degrees, a car's colors, or phones. Chen Model. It does so my representing the connection with double lines

What are entities that participate in a relationship known as?

PARTICIPANTS! Each relationship is identified by a name that describes the relationship. The relationship name is an active or passive verb; for example, a STUDENT takes a CLASS, a PROFESSOR teaches a CLASS, a DEPARTMENT employs a PROFESSOR, a DIVISION is managed by an EMPLOYEE, and an AIRCRAFT is flown by a CREW.

When loading tables which tables should be loaded first to avoid referential integrity errors?

The 1 side of a 1:M relationship, regardless of whether or not the relationship is weak or strong.

What are the 3 main components represented in an ERD?

The dbs main components are represented: entities, attributes and relationships.

What does Relationship Degree refer to?

The number of entities involved in a relationship.

What makes a weak relationship?

When the parent entity's PK is not part of the child entities' PK.

How are strong and weak relationships represented in Chen Notation?

They aren't! Chen Notation is conceptual and doesn't concern itself about such things.

In terms of keys what defines a strong (identifying) relationship?

When the PK of the child entity contain a PK component of the parent entity.

What makes a strong relationship?

When the parent entities PK is part of the child entities PK.