Description
how to create entity relationship diagrams with respect to attributes, identifiers, cardinality of relationships, cardinality constraints.
1
?
Entities:
? Entity instance–person, place, object, event, concept (often corresponds to a row in a table) ? Entity Type–collection of entities (often corresponds to a table)
?
Relationships:
? Relationship instance–link between entities (corresponds to primary key-foreign key equivalencies in related tables) ? Relationship type–category of relationship…link between entity types
?
Attribute–property or characteristic of an entity or relationship
type (often corresponds to a field in a table)
2
Sample E-R Diagram
3
Basic E-R notation
Entity symbols
Attribute symbols
A special entity that is also a relationship
Relationship degrees specify number of entity types involved
Relationship symbols
Relationship cardinalities specify how many of each entity type is allowed
4
?
SHOULD BE:
? An object that will have many instances in the database ? An object that will be composed of multiple attributes ? An object that we are trying to model
?
SHOULD NOT BE:
? A user of the database system ? An output of the database system (e.g., a report)
5
Figure 3-4 Example of inappropriate entities
System user
Inappropriate entities
System output
Appropriate entities
6
? ?
Attribute–property or characteristic of an entity or relationship type Classifications of attributes:
? ? ? ? ? Required versus Optional Attributes Simple versus Composite Attribute Single-Valued versus Multi valued Attribute Stored versus Derived Attributes Identifier Attributes
7
?
? ?
Identifier (Key)–An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Simple versus Composite Identifier Candidate Identifier–an attribute that could be a key…satisfies the requirements for being an identifier
8
? Degree
of a relationship is the number of entity types that participate in it
?Unary Relationship ?Binary Relationship ?Ternary Relationship
15
Degree of relationships – from Figure 3-2
One entity related to another of the same entity type
Entities of two different types related to each other
Entities of three different types related to each other
16
?
One-to-One
?
One-to-Many
? Each entity in the relationship will have exactly one related entity
? An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity ? Entities on both sides of the relationship can have many related entities on the other side
?
Many-to-Many
17
?
?
Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity Minimum Cardinality Maximum Cardinality
? The maximum number ? If zero, then optional ? If one or more, then mandatory
?
18
Figure 3-12 Examples of relationships of different degrees a) Unary relationships
19
Figure 3-12 Examples of relationships of different degrees (cont.) b) Binary relationships
20
Figure 3-12 Examples of relationships of different degrees (cont.) c) Ternary relationship
Note: a relationship can have attributes of its own
21
Figure 3-17 Examples of cardinality constraints
a) Mandatory cardinalities
A patient history is recorded for one and only one patient
A patient must have recorded at least one history, and can have many
22
Figure 3-17 Examples of cardinality constraints (cont.)
b) One optional, one mandatory
A project must be assigned to at least one employee, and may be assigned to many
An employee can be assigned to any number of projects, or may not be assigned to any at all
23
Figure 3-17 Examples of cardinality constraints (cont.)
a) Optional cardinalities
A person is is married to at most one other person, or may not be married at all
24
Figure 3-21 Examples of multiple relationships a) Employees and departments
Entities can be related to one another in more than one way
25
Figure 3-21 Examples of multiple relationships (cont.) b) Professors and courses (fixed lower limit constraint)
Here, min cardinality constraint is 2
26
?
An
?
?
A
When should a relationship be an associative entity?
relationship–links entities together
entity–has attributes
with attributes instead
? All relationships for the associative entity should be many ? The associative entity could have meaning independent of the other entities ? The associative entity preferably has a unique identifier, and should also have other attributes ? The associative entity may participate in other relationships other than the entities of the associated relationship ? Ternary relationships should be converted to associative entities
30
Figure 3-11a A binary relationship with an attribute
Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship
31
Figure 3-11b An associative entity (CERTIFICATE)
Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right.
Note that the many-to-many cardinality between entities in Figure 3-11a has been replaced by two one-to-many relationships with the associative entity.
32
Figure 3-13c An associative entity – bill of materials structure
This could just be a relationship with attributes…it’s a judgment call
33
Microsoft Visio Notation for Pine Valley Furniture E-R diagram
Different modeling software tools may have different notation for the same constructs
35
doc_838701994.pptx
how to create entity relationship diagrams with respect to attributes, identifiers, cardinality of relationships, cardinality constraints.
1
?
Entities:
? Entity instance–person, place, object, event, concept (often corresponds to a row in a table) ? Entity Type–collection of entities (often corresponds to a table)
?
Relationships:
? Relationship instance–link between entities (corresponds to primary key-foreign key equivalencies in related tables) ? Relationship type–category of relationship…link between entity types
?
Attribute–property or characteristic of an entity or relationship
type (often corresponds to a field in a table)
2
Sample E-R Diagram
3
Basic E-R notation
Entity symbols
Attribute symbols
A special entity that is also a relationship
Relationship degrees specify number of entity types involved
Relationship symbols
Relationship cardinalities specify how many of each entity type is allowed
4
?
SHOULD BE:
? An object that will have many instances in the database ? An object that will be composed of multiple attributes ? An object that we are trying to model
?
SHOULD NOT BE:
? A user of the database system ? An output of the database system (e.g., a report)
5
Figure 3-4 Example of inappropriate entities
System user
Inappropriate entities
System output
Appropriate entities
6
? ?
Attribute–property or characteristic of an entity or relationship type Classifications of attributes:
? ? ? ? ? Required versus Optional Attributes Simple versus Composite Attribute Single-Valued versus Multi valued Attribute Stored versus Derived Attributes Identifier Attributes
7
?
? ?
Identifier (Key)–An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type Simple versus Composite Identifier Candidate Identifier–an attribute that could be a key…satisfies the requirements for being an identifier
8
? Degree
of a relationship is the number of entity types that participate in it
?Unary Relationship ?Binary Relationship ?Ternary Relationship
15
Degree of relationships – from Figure 3-2
One entity related to another of the same entity type
Entities of two different types related to each other
Entities of three different types related to each other
16
?
One-to-One
?
One-to-Many
? Each entity in the relationship will have exactly one related entity
? An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity ? Entities on both sides of the relationship can have many related entities on the other side
?
Many-to-Many
17
?
?
Cardinality Constraints - the number of instances of one entity that can or must be associated with each instance of another entity Minimum Cardinality Maximum Cardinality
? The maximum number ? If zero, then optional ? If one or more, then mandatory
?
18
Figure 3-12 Examples of relationships of different degrees a) Unary relationships
19
Figure 3-12 Examples of relationships of different degrees (cont.) b) Binary relationships
20
Figure 3-12 Examples of relationships of different degrees (cont.) c) Ternary relationship
Note: a relationship can have attributes of its own
21
Figure 3-17 Examples of cardinality constraints
a) Mandatory cardinalities
A patient history is recorded for one and only one patient
A patient must have recorded at least one history, and can have many
22
Figure 3-17 Examples of cardinality constraints (cont.)
b) One optional, one mandatory
A project must be assigned to at least one employee, and may be assigned to many
An employee can be assigned to any number of projects, or may not be assigned to any at all
23
Figure 3-17 Examples of cardinality constraints (cont.)
a) Optional cardinalities
A person is is married to at most one other person, or may not be married at all
24
Figure 3-21 Examples of multiple relationships a) Employees and departments
Entities can be related to one another in more than one way
25
Figure 3-21 Examples of multiple relationships (cont.) b) Professors and courses (fixed lower limit constraint)
Here, min cardinality constraint is 2
26
?
An
?
?
A
When should a relationship be an associative entity?
relationship–links entities together
entity–has attributes
with attributes instead
? All relationships for the associative entity should be many ? The associative entity could have meaning independent of the other entities ? The associative entity preferably has a unique identifier, and should also have other attributes ? The associative entity may participate in other relationships other than the entities of the associated relationship ? Ternary relationships should be converted to associative entities
30
Figure 3-11a A binary relationship with an attribute
Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship
31
Figure 3-11b An associative entity (CERTIFICATE)
Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right.
Note that the many-to-many cardinality between entities in Figure 3-11a has been replaced by two one-to-many relationships with the associative entity.
32
Figure 3-13c An associative entity – bill of materials structure
This could just be a relationship with attributes…it’s a judgment call
33
Microsoft Visio Notation for Pine Valley Furniture E-R diagram
Different modeling software tools may have different notation for the same constructs
35
doc_838701994.pptx