CPS 430/542 Lecture notes: E/R Modeling Constraints and Weak Entity Sets

Coverage: [FCDB] §§2.3-2.4 (pp. 47-59)

Solved exercise 2.1.1 on p. 36 from [FCDB]

special-case entity sets (or subclasses)
use an `isa' relationship
- represented with a triangle (half diamond) which points to superclass
- always one-one
entity belongs to both entity sets
subclasses inherit the attributes of their ancestors and participate in their relationships
example:
- cartoons have voices
- murder mysteries have weapons
what about multiple inheritance?
- e.g., Roger Rabbit
- not every situation can be modeled with an E/R diagram

a directed graph with no cycles, e.g., a C++ class inheritance hierarchy involving multiple inheritance
in a tree, there is only one path from the root to each leaf
every tree is also a DAG, but
some DAGs are not trees
how can we model these types of graphs using the E/R model?

additional requirements some of which the modeling language can enforce and others for which it cannot
reflection of schema, not contents
examples
single-valued constraints
- attributes may only have a single value
- no way to indicate the absence of an attribute value
- many-one relationships
keys
referential integrity
domain constraints

attribute or set of attributes from an entity class which uniquely identifies an entity
two members of an entity set may not have the same value for a key, e.g., ssn
underline key attributes in E/R
can consist of more than one attribute
there may be more than one key
- but no way to represent this in E/R
- choose one
sometimes key attributes do not belong to entity set: weak entity sets (coming soon ...)
storage implications, e.g., C int vs. linked list of ints analogy

strengthens at most one to exactly one
use curved arrow head in E/R ---)
---) = ---> + `referenced entity must exist', i.e., you can only add a curved arrow head if you already have a regular arrow head
movie-studios example
enforcement
- prevent deletion of referenced entity (in [Studios]) until no referents (in [Movies]) refer to it
- if referenced entity deleted, all referent entities are deleted as well
- force the entry or association of a referenced entity when inserting a member of the referent entity set
3 ways to enforce
- 2 involve deletions
- 1 involves insertion
extended example
- studios can exist without a president,
- but you cannot call yourself a president if you are not the president of a studio

attributes
- types (int, floats)
- ranges
  - ≤ 4.0 (gpa)
  - ≤ 25 (baseball roster)
cardinality on relationships
write `Note(s): ...' on the margin of the page for aspects which cannot be represented using the E/R model

an entity set which derives a subset of its key attributes from other entity sets
sources of weakness
- hierarchy of entity sets (not inheritance)
- connecting entity sets resulting from `pushing out', i.e., those with no attributes
a strong entity set must live at the end of a chain of weak sets
gets its key from
- zero or more of its attributes, and
- key attributes from entity sets which are reached by certain many-one, called supporting, relationships
- weak entity set may participate in non-supporting relationships
representation in E/R
- use double rectangle [[...]] for weak entity set
- use double diamond <<...>> for supporting relationship

entity set [[Crews]] is weak (a weak entity set)
relationship <<works for>> is a supporting relationship
key of entity set [[Crews]] consists of its attribute (number) and the key attribute (name) from entity set [Studios]
entity set [[Crews]] gets part of its key from the key of entity set [[Studios]]

binary
many-one (includes one-one)
referential integrity
attributes supplied from referenced entity set F to help form key for weak entity set E must be key attributes of F
if referenced entity set F is weak, following the chain to a non-weak entity set
rationale behind requirements, especially binary, many-one, and referential integrity requirements