Lec 4 Advanced ER Modeling

Connectivity

The shape of the relationship

Cardinality

The exact minimum and maximum

(0, N)

Optional, no upper limit

(1, N)

Mandatory, no upper limit

(1, 1)

Exactly one, always

(0, 1)

At most one, possibly none

Existence dependence

The entity cannot exist without another one

Key dependence

Its own attributes cannot identify it

Weak entity

Both are true, so the parent's key becomes part of its primary key

Not weak

If either test fails, it is strong entity that happens to be mandatory

Recursive relationship

One entity type, related to itself

Ternary relationship

Three entities in one relationship, and when to split it

Specialization

Direction: top down

You start with: one general entity

You notice: some instances have extra attributes

Example: person splits into STUDENT and EMPLOYEE

Generalization

Direction: bottom up

You start with: several specific entities

You notice: several entities share attributes

Example: FACULTY and ADMIN_STAFF share EMPLOYEE

Disjoint

An instance is in at most one subtype

Overlapping

An instance may be in several

Total

Every supertype instance is in a subtype

Partial

Some may be in none

Entity clustering

A readability device for large models

Grouping related entities into a named ___ shown as one box at the top level

Fan trap

Two branches from one hub, and the answer ___ out

Chasm trap

A path that looks complete and is not