last updated 5-sep-12
Goal of design is to generate a formal specification of the database schema
E-R Model is not SQL based. It's not limited to any particular DBMS. It is a conceptual and semantic model – captures meanings rather than an actual implementation
The E-R Model: The enterprise is viewed as set of
Symbols used in E-R Diagram
Entity: an object that is involved in the enterprise and that be distinguished from other objects. (not shown in the ER diagram--is an instance)
Entity Type: set of similar objects or a category of entities; they are well defined
Attribute: describes one aspect of an entity type; usually [and best when] single valued and indivisible (atomic)
An entity type is named and is described by set of attributes
Domain: possible values of an attribute.
Key: subset of attributes that uniquely identifies an entity (candidate key)
The meta-information of entity type name, attributes (and associated domain), key constraints
Entity Types tend to correspond to nouns; attributes are also nouns albeit descriptions of the parts of entities
May have null values for some entity attribute instances – no mapping to domain for those instances
Superkey: an attribute or set of attributes that uniquely identifies an entity--there can be many of these
Composite key: a key requiring more than one attribute
Candidate key: a superkey such that no proper subset of its attributes is also a superkey (minimal superkey – has no unnecessary attributes)
Primary key: the candidate key chosen to be used for identifying entities and accessing records. Unless otherwise noted "key" means "primary key"
Alternate key: a candidate key not used for primary key
Secondary key: attribute or set of attributes commonly used for accessing records, but not necessarily unique
Foreign key: term used in relational databases (but not in the E-R model) for an attribute that is the primary key of another table and is used to establish a relationship with that table where it appears as an attribute also.
So a foreign key value occurs in the table and again in the other table. This conflicts with the idea that a value is stored only once; the idea that a fact is stored once is not undermined.
Rectangle -- Entity
Ellipses -- Attribute (underlined attributes are [part of] the primary key)
Double ellipses -- multi-valued attribute
Dashed ellipses-- derived attribute, e.g. age is derivable from birthdate and current date.
[Drawing notes: keep all attributes above the entity. Lines have no arrows. Use straight lines only]
Relationship: connects two or more entities into an association/relationship
Relationship Type: set of similar relationships
Relationship Types may also have attributes in the E-R model. When they are mapped to the relational model, the attributes become part of the relation. Represented by a diamond on E-R diagram.
Relationship types can have descriptive attributes like entity sets
Relationships tend to be verbs or verb phrases; attributes of relationships are again nouns
[Drawing tips: relationship diamonds should connect off the left and right points; Dia can label those points with cardinality; use Manhattan connecting line (horizontal/vertical zigzag)]
An attribute of a relationship type adds additional information to the relationship
The role of a relationship type names one of the related entities. The name of the entity is usually the role name.
e.g., "John" is value of Student role, "CS" value of Department role of MajorsIn relationship type
(John, CS, 2000) describes a relationship
Problem: relationships can relate elements of same entity type
e.g., ReportsTo relationship type relates two elements of Employee entity type:
- Bob reports to Mary since 2000
We do not have distinct names for the roles. It is not clear who reports to whom.
Solution: the role name of relationship type need not be same as name of entity type from which participants are drawn
Optional to name role of each entity-relationship, but helpful in cases of
Roles are edges labeled with role names (omitted if role name = name of entity set). Most attributes have been omitted.
Relationship types are described by the set of roles (entities) and [optional] attributes
Think that entities are nouns; relationship types are often verbs
Here we have equate the role name (Student) the name of the entity type (Student) of the participant in the relationship.
The number of roles in the relationship
Binary – links two entity sets; set of ordered pairs (most common)
Ternary – links three entity sets; ordered triples (rare). If a relationship exists among the three entities, all three must be present
N-ary – links n entity sets; ordered n-tuples (very rare). If a relationship exists among the entities, then all must be present. Cannot represesnt subsets.
Note: ternary relationships may sometimes be replaced by two binary relationships (see book Figures 3.5 and 3.13). Semantic equivalence between ternary relationships and two binary ones are not necessarily true.
Cardinality is the number of entity instances to which another entity set can map under the relationship. This does not reflect a requirement that an entity has to participate in a relationship. Participation is another concept.
One-to-one: X-Y is 1:1 when each entity in X is associated with at most one entity in Y, and each entity in Y is associated with at most one entity in X.
One-to-many: X-Y is 1:M when each entity in X can be associated with many entities in Y, but each entity in Y is associated with at most one entity in X.
Many-to-many: X:Y is M:M if each entity in X can be associated with many entities in Y, and each entity in Y is associated with many entities in X ("many" =>one or more and sometimes zero)
Existence dependency: Entity Y is existence dependent on entity X is each instance of Y must have a corresponding instance of X
In that case, Y must have total participation in its relationship with X
If Y does not have its own candidate key, Y is called a weak entity, and X is strong entity
Weak entity may have a partial key, called a discriminator, that distinguishes instances of the weak entity that are related to the same strong entity
Use double rectangle for weak entity, with double diamond for relationship connecting it to its associated strong entity
Note: not all existence dependent entities are weak – the lack of a key is essential to definition
Contains the following features:
Role names, Ri, and their corresponding entity sets. Roles must be single valued (the number of roles is called its degree)
Attribute names, Aj, and their corresponding domains. Attributes in the E-R model may be set or multi-valued.
Key: Minimum set of roles and attributes that uniquely identify a relationship
Relationship: <e1, …en; a1, …ak>
- ei is an entity, a value from Ri’s entity set
- aj is a set of attribute values with elements from domain of Aj