Database Systems Final - ER diagram

What is a strong entity

An entity with its own unique primary key; exists independently

What is a weak entity

An entity that cannot exist without a strong entity and uses part of the strong entity’s key

What are the 3 notati8ons used to show a weak entity

Double rectangle (weak entity)

Double diamond (identifying relationship)

Partial identifier (dashed underline)

What is an identifying relationship

A relationship that connects a weak entity to its strong entity and participates in the weak entity’s primary key

When do you model an associative entity

When converting a many-to-many (M: N) relationship that has attributes

What does an associative entity do

Turns an M: N relationship into a 1: M relationships

both diagrams with/ without associative entities model M: N?

Yes, both are valid representations of a many-to-many relationship

What is a multivalued attribute

An attribute with more than one value (shown with double oval)

What is a composite attribute

An attribute that can be broken into smaller parts (First/ Last Name)

What is a derived attribute

An attribute calculated from other attributes (shown with dashed oval)

What does “mandatory one” mean

The entity must participate, and only once

What does “mandatory many” mean

The entity must participate at least once, and possible many times

What does “optional one” mean

Participation is optional, and if it occurs, it occurs once

What does “optional many” mean

Participation is optional (may be zero) and can be many

What does “at most” mean

Maximum number allowed (at most 3 = 0-3)

What does “at least” mean

Minimum number allowed (at least one = one or more)

What does “exactly” mean

Patriation must be that specific number

What is a unary relationship

A relationship with one entity related to itself

What is a binary relationship

A relationship involving two entities

What is a ternary relationship

A relationship involving three entities

In the STUDENT - COURSE example, what entity represents the many to many

Enrolls (associative entity)

What is an attribute that would belong to the associative entity ENROLLS

Grade

What is a non-identifying relationship

A relationship that does not help form the primary key of the child entity

How do you know an associative entity is needed

When an M: N relationship has attributes OR when you want to avoid direct M: N relationships

In ER modeling, what does the circle (O) represent

Optional participation

What doe the striaght line (I) symbol represent

Mandatory participation

What is the symbol for mandatory many

|<

What is the symbol for optional one

O-

Where should composite attributes subparts appear in the ER diagram

Inside connected ovals branching from the composite attribute

What is a partial identifier used for

Identifying a weak entity along with the strong entity’s key