ER models¶

The Entity-Relationship Model¶

Meta Object Facility(MOF)¶

\(M_0\) : the real world 数据

\(M_1\) : model 元数据

\(M_2\) : metamodel 语言

\(M_3\) : metametamodel 元语言

represent the overall conceptual structure of a database

Three basic concepts¶

1. entity sets(实体集) & attributes(属性):
   1. Entity: an object that exists and is distinguishable from other objects
   2. Entity set: a set of entities of the same type that share same properties
2. relationship sets(联系集)
   1. Relationship: an association among several entities
   2. Relationship set: a mathmatical relation among \(n \geq 2\) entity sets \(\{(e_1, \cdots, e_n) \mid e_1 \in E_1, \cdots, e_n \in E_n\}\)
3. Attributes: a set of attributes describe an entity
   1. simple/composite; single-valued/multivalued; derived(dynamic from other attribute);
   2. domain
   3. super key: one or more attributes whose value uniquely determine each entity
   4. candidate key: minimal super key
   5. primary key: one of the candidate keys that is selected

Symbols of ER-model¶

Entity:

Entity set:

Complex attributes:

{...} means multiple value attribute, ...() means derived attribute

Relationship set:

(binary)

The box below is an "attribute set"

(multiple)

(multiple occurance, add a role)

mapping cardinality¶

Four types:

• one to one
• one to many
• many to one
• many to many

arrow means: (the other side) can only appear once

Participation¶

double line means total partipation

Weak Entity Sets¶

We use extra attributes called discriminator to uniquely identify a weak entity.

double line frame, double line relationship set

mostly: all participation, one-to-many

dot lines to mark the primary key

Reduction to Relation Schemas¶

We can reduce entity sets and relationship sets uniformly as relation schemas

• Strong Entity set: to a schema with the same attributes
• Weak Entity set: to a schema include a column for the primary key and other attributes
• Many-to-Many relationship set: to a schema including primary keys of the two particiating entity sets
• Composite attributes: flattened
• Multivalued attributes: add a new schema that add this value as a primary key

Redundancy of Schemas¶

one-one / one-many / many-one relationship: "many" can be added to the "one" side as extra attributes

The schema corresponding to a relationship set linking a weak entity set to its identifying string entity set is redundant

Advanced Topic¶

Redundant attributes¶

Specialization & Generation¶

OOP extension

Aggregation¶

Aggregate some entities and relationship as a whole

Design Issues¶

entity set v.s. attributes:¶

entity sets v.s. relational sets¶

relational sets are used to describe an action that occurs between entities

binary v.s. non-binary realtionships¶

UML: unifying modeling language¶