Chapter 2 Flashcards
(90 cards)
1
Q
What is an entity?
A
- Entity instance - person, place, object, event, concept (often corresponds to a row in a table)
- Entity type - collection of entitities (often corresponds to a table)
2
Q
What are relationships?
A
Relationship instance - link between entities (corresponds to primary key-foreign key equivaltencies in related tables)
Relationsihp type - category of relationship.. link between entity types
3
Q
What are attributes?
A
Properties or characteristics of an entity or relationship type (often corresponds to a field in a table)
4
Q
Sample E-R Diagram
A
5
Q
Basic E-R notation (visualized)
A
6
Q
What are business rules?
A
Statements that define or constrain some aspect of the business.
7
Q
What are business rules derived from?
A
Policies, procedures, events, and functions
8
Q
What are some aspects of business rules?
A
- Assert business structure
- Control/influence business behavior
9
Q
How are business rules expressed/automated?
A
- They are expressed in terms that are familiar to end users
- Automated through DBMS software
10
Q
What are the components of a good business rule?
A
- Declarative
- Precise
- Atomic
- Consistent
- Expressable
- Distinct
- Business-oriented
11
Q
Business Rules - What does declarative mean?
A
What, not how
12
Q
Business Rules - What does precise mean?
A
Clear, agreed-upon meaning
13
Q
Business rules - What does atomic mean?
A
One statement
14
Q
Business rules - What does consistent mean?
A
internally and externally consistent
15
Q
Business Rules - What does expressible mean?
A
Structured, natural language
16
Q
Business Rules - What does distinct mean?
A
non-redundant
17
Q
Business Rules - What does business-oriented mean?
A
Understood by business people
18
Q
What are the components of a good data name?
A
- Related to business, not technical, characteristics
- Meaningful and self-documenting
- Unique
- Readable
- Composed of words from an approved list
- Repeatable
- Written in standard syntax
19
Q
What is a data definition?
A
Explanation of a term or fact
20
Q
What is a term?
A
word or phrase with specific meaning
21
Q
What is a fact?
A
association between two or more terms
22
Q
What are the guidelines for good data definition?
A
- Concise description of essential data meaning
- Gathered in conjunction with systems requirements
- Accompanied by diagrams
- Achieved by consensus, and iteratively refined
23
Q
What is an entity?
A
a person, place, object, event, or concept in the user environment about which the organization wishes to maintain data
24
Q
What is an entity type?
A
A collection of entities that share common properties or characteristics.
25
What is an entity instance?
A single occurrence of an entity type
26
Entity type/instance example
27
What should be an entity?
- 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
28
What should an entity not be?
- A user of the database system
- An output of the database system (e.g. a report)
29
Example of Appropriate/Innappropriate entities
30
What are characteristics of a strong entity?
- Exists independently of other types of entities
- Has it's own unique identifier (identifiter underline with single line)
31
What are the characteristics of a weak entity?
- Dependent on a strong entity (identifying owner), cannot exists on its own
- Does not have a unique identifier (only a partial identifier)
- Entity box and partial identifier have double lines
32
What is an identifying relationship in relation to entities?
- Links strong entities to weak entities
33
Stong vs. Weak entity visualized
34
What are the guidelines for naming entities?
- Singular Noun
- Specific to organization
- Concise, or abbreviation
- For event entities, the result not the process
- Name consistent for all diagrams
35
What are the guidelines for defining entities?
- "an X is"
- Describe unique characteristics of each instance
- Explicit about what is and is not the entity
- When an instance is created or destroyed
- Changes to other entity types
- History that should be kept
36
What is an attribute?
Property or characteristic of an entity or relationship type
37
What are the classifications of attributes?
- Required vs Optional
- Simple vs Composite
- Single-valued vs Multivalued
- Stored vs Derived
- Identifier
38
Example of Required vs Optional Attributes
39
What is a required attribute?
Must have a value for every entitiy (or relationship) instance with which it is associated
40
What is an optional attribute?
May not have a value for every entity (or relationship) instance with which it is associated.
41
What is a composite attribute?
An attribute that has meaningful component parts (attributes)
42
Composite attribute (example)
43
What is a multivalued attribute?
May take on more than one value for a given entity (or relationship) instance
44
What is a derived attribute?
Values can be calculated from related attribute values (not physically stored in the database)
45
Multivalued and Derived attribute example
46
What is an identifier (Key)?
An attribute (or combination of attributes) that uniquely identifies individual instances of an entity type
47
What are the two types of identifiers?
- Simple
- Composite
48
What is a candidate identifier?
An attribute that could be an identifier (satisfies the requirements for being an identifier)
49
What are two criteria for choosing identifiers?
- Will not change in value
- Will not be null
50
What should you avoid in identifiers?
- Intelligent identifiers (containing locations or people that might changed, basically information that might change)
51
What should you do for long composite keys?
Substitute new simple keys
52
Simple and Composite Identifier example
53
What requirements are there when naming attributes?
- Name should be a singular noun or noun phrase
- Name should be unique
- Name should follow a standard format
- Similar attributes of different entity types should use the same qualifiers and classes
54
What are the requirements when defining attributes?
- State what the attribute is and possibly why it is important
- Make it clear what is and is not included in the attribute's value
- Include aliases in documentation
- State source of values
- State whether attribute value can change once set
- Specify required vs. optional
- State min and max number of occurences allowed
- Indicate relationships with other attributes
55
What is the difference between relationship types and relationship instances?
- Relationship type is modeled as lines between entity types
- Relationship Instance is between specific entity instances
56
What is the difference between relationship types and relationship instances?
- Relationship types are modeled as lines between specific entity types
- Relationship instances are between specific entity instances
57
Can relationships have attributes?
- Yes, these describe features pertaining to the associations between the entities in the relationship.
58
Can entities have more than one relationship between them?
- Yes, these are call multiple relationships
59
What is an associative entity?
Combination of relationship and entity
60
Relationship types and instance visualized
61
62
What is the degree of a relationship?
The number of entity types that participate in it
63
What are the three types of relationships?
- Unary
- Binary
- Ternary
64
Relationship degrees visualized
65
What are the cardinality of relationships?
- One-to-one
- One-to-many
- Many-to-Many
66
What is a one-to-one relationship cardinality?
Each entity in the relationship will have exactly one related entity
67
What is a one-to-many relationship cardinality?
- 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
68
What is a many-to-many relationship cardinality?
Entities on both sides of the relationship can have many related entities on the other side
69
Unary relationship examples
70
Binary relationship example
71
Ternary relationship example
72
What are cardinality Constraints?
The number of instances of one entity that can or must be associated with each instance of another entity.
73
What are the minimum cardinalities?
- If zero, then optional
- If one or more, then mandatory
74
75
What is the maximum cardianality?
The maximum number
76
Example of cardinality constraints
77
Examples of cardinality constraints, optional and mandatory
78
Examples of cardinality constraints, all optional
79
Examples of mulitple relationships
80
Example of multiple relationships, fixed lower limit constraint
81
Simple and Composite attributes shown as relationships
82
What does an entity have?
Attributes
83
What does a relationship do?
Links Entities together
84
When should a relationship with attributes instead be an associative entity?
- When all relationships for the associative entity should be many
- When the associative entity could have meaning independent of the other entitiies
- When the associative entity preferably has a unique identifier, and should also have other attributes
- The associative entity may participate in other relationships other than th entities of the associated relationship
- Ternary relationships should be converted to associative entities
85
Binary relationship with an attribute example
86
What is an associative entity similar to?
A relationship with an attribute.
87
Associative entity example
88
Good example of ternary relationship and business rules
89
What is a time stamp?
a time value that is associated with a data value, often indicating when some event occured that affected the data value
90
