Week 3

Question 1

What are the different ERD components?

Answer 1

• Entities: Corresponds to an entire table, not row and is represented by a rectangle
• Attributes: Correspond to columns in tables and are listed in rectangle (in entity)
• Relationships: Reflected by P.K and F.K pairs and are represented by lines among entities.

Question 3

What are the 4 steps to creating an ERD?

Answer 3

1. Identify entities through business rules
2. Identify relationships among the entities
3. Specify attributes within each entity
4. Check/specify all the details (e.g. weak entity)

Question 4

What is Connectivity?

Answer 4

Connectivity describes relationship classification (e.g. 1:1, 1:M, M:N)

Question 5

What is Cardinality?

Answer 5

Cardinality expresses the number of entity occurrences associated with one occurrence of a related entity. (e.g. one employee can be associated with a maximum of two tablets)

Question 6

What are the two different types of relationship participation?

Answer 6

• Optional participation: One entity occurrence does not require corresponding entity occurrence in particular relationship
• Mandatory participation: One entity occurrence requires corresponding entity occurrence in particular relationship.

Question 7

What are the general rules for FK: 1:1 relationships?

Answer 7

The general rules for FK in 1:1 relationships are:

• Put the Foreign Key in the strong entity or in the most frequently accessed entity
• Or put foreign key in the entity selected by the semantics of the problem.

Question 8

What are the general rules for FK: 1:M relationships?

Answer 8

Create the foreign key by putting the primary key of the “1” in the table of the “M”.

Question 9

What are the general rules for FK: M:N relationships?

Answer 9

Convert the M:N relationship to a composite (bridge) entity consisting of (at least) the parent tables’ primary keys.

Question 10

What is a composite entity?

Answer 10

Composite entities (aka bridge tables) are used to bridge between M:N relationships. They are composed of the primary keys of each entity needing connection. Normally contains additional attributes

Question 11

What is a primary key?

Answer 11

A primary key is an essential attribute of each entity that is unique to the record within that entity.

Question 12

What is the difference between a simple and a composite attribute?

Answer 12

Simple attribute: Can’t be subdivided (e.g. Age, Sex, Marital Status)

Composite Attribute: Can be subdivided into additional attributes (e.g. Address into street, city, zip)

Question 13

What is the difference between a single-valued and multivalued attribute?

Answer 13

Single-valued attribute: Can only have a single value (e.g. person can only have one social security number)

Multivalued attribute: Can have many values (e.g. Person may have several college degrees)

Question 14

How do you handle multivalued attributes?

Answer 14

If there is a multivalued attribute there are two options:

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

Question 15

What are derived attributes?

Answer 15

Derived attributes are attributes that can be derived with an algorithm (computation) (e.g. age can be derived from date of birth). If the derived attribute is often queried upon, keep it in the table.

Question 16

What is existence dependancy?

Answer 16

If an entity’s existence depends on the existence of one or more other entities, it is said to be existence dependent (e.g. a CLASS is existence dependent on COURSE)

Question 17

What are the two different types of relationship strength?

Answer 17

• Weak (non-identifying) relationships
  Exist if PK of related entity does not contain PK component of parent
• Strong (Identifying) relationships
  Exists when PK of related entity contains PK component of parent entity

Question 18

What is a weak entity?

Answer 18

A weak entity is an entity whose existence depends on another entity and has a primary key that is partially or totally derived from parent entity. (A weak entity is connected by a strong relationship with its parents entity)

Question 19

What are the three different types of relationship degrees?

Answer 19

There are three different relationship degrees:
1. Unary: Single entity with a recursive relationship

2. Binary: Two entities associated
3. Ternary: Three entities associated

Question 20

What is a recursive entity?

Answer 20

A recursive entity is one in which a relationship can exist between occurrence of the same entity set. A recursive entity is found within a unary relationship.

Question 21

What is a strong entity?

Answer 21

A strong entity is an entity whose existence is not dependent on any other entity. Strong entities have their own attributes that uniquely identify them