SAD chapter 6
What is Cardinality?
the number of instances of one entity that can or must be associated with each instance of another entity o One instance in an entity refers to one and only one instance in the related entity (1:1) o One instance in an entity refers to one or more instances in the related entity (1:N) o One or more instances in an entity refer to one or more instances in the related entity (M:N)
What is a Foreign Key?
A foreign key is a primary key of one entity that is contributed to (duplicated in) another entity for the purpose of identifying instances of a relationship. A foreign key (always in a child entity) always matches the primary key (in a parent entity). A relationship implies that instances of one entity are related to instances of another entity The primary key of one entity is migrated into the other entity as aforeign key. The foreign key in the child entity is what enables you to "link back" to the correct parent in the parent entity
How do you add appropriate attributes? (Step 2)
Identify attributes of the entity that are relevant to the system under development. Select the entity's candidate identifier (final decision may be postponed until Design phase)
How do you identify entities for step 1?
Identify major categories of information o If available, check the process models for data stores, external entities, and data flows - these indicate the kinds of information that are captured and flow through the system. o Check the major inputs and outputs from the use cases Verify that there is more than one instance of the entity that occurs in the system
Why Is Data Modeling Crucial?
- Data is a resource to be shared by as many processes as possible. - Data organization must be flexible and adaptable to unanticipated business requirements - and that is the purpose of data modeling.
Example of something that is not an entity?
- If Police arrest you and bring you to jail, how many jails are there? If only one, you don't need to capture information about it.
What are attributes in regards to ERD? How are they named?
- Information captured about an entity - Only those used by the organization should be included in the model - Attribute names are nouns - Sometimes entity name is added at the beginning of the attribute name for clarity
What is normalization?
- The process analysts use to validate data models. - Series of rules applied to logical data model to improve its organization - Three normalization rules are common
What can ERD symbols show? (3)
1. when the existence of an entity instance is optional for a related entity instance 2. when one instance of an entity can be related to only one or to many instances of another entity 3. when one instance of an entity must exist for an instance of another to exist
What is an entity? What determines if something is an entity?
A person, place, event, or thing about which data is collected Must be multiple occurrences to be an entity
What is a Composite attribute?
An attribute that has meaningful component parts. An address is composed of street, city etc.
What is a Relationship? What are the first and second called? What should their name be made of? Which direction?
Associations between entities The first entity in the relationship is the parent entity; the second entity in the relationship is the child entity Relationships should have active verb names Relationships go in both directions
What is a derived attribute?
Calculated from other values GPA
What are business rules?
Constraints that are followed when the system is in operation
What generally corresponds to entities?
Data stores of the DFD
What is the question to check 2nd Normal form? What do you do if the answer is yes? No?
Do any attribute values depend on an attribute that is not the entity's identifier? - Remove the transitive dependency or derived attribute. - Move the attributes to an entity in which their values are dependent on the identifier. - Typically a relationship will be needed to connect the old and new entities Else: The data model is in 3NF
What is the question to check 0 normal form? What do you do if the answer is yes? No?
Do any attributes have multiple values for a single instance of an entity? - Remove the repeating attributes and repeating groups. - Create an entity that describes the attributes - Typically a relationship will be needed to connect the old and new entities Else: The data model is in 1NF
What entities should not be included initially?
Entities associated with implementation of the system. (archive files of older data). They will be added later.
What is an ERD?
Entity Relationship Diagram (ERD) o A popular way to depict the data model
Are Data models faster or slower than process models to make?
Faster, and are much smaller
What does constructing a data model help do?
Helps analysts and users quickly reach consensus on business terminology and rules.
What is an insert anomaly? How to resolve?
If inserting a new instance of an entity is dependent on a primary key of another entity. We would have an incomplete instance with missing data. Normalize to fix.
What is an update anomaly? How to resolve?
If there are multiple copies of certain entities and a value must be updated, then either you have to search all instances of database and update them all (Expensive), or some are missed and you end up with inconsistent data. Normalize and prevent duplicates from existing
What is the question to check 1st Normal form? What do you do if the answer is yes? No?
Is the identifier composed of more than one attribute? If so, are any attribute values dependent on just part of the identifier? - Remove the partial dependency. - Move the attributes to an entity in which their values are dependent on the entire identifier. - Typically a relationship will be needed to connect the old and new entities Else: The data model is in 2NF
What type of process is drawing the ERD?
Iterative process of trial and revision
What is a Multi-valued attribute?
May be more than one allowed. Someone can have 1 or more majors
Data structures and properties are reasonably permanent. Therefor...
More stable than the processes that use the data
What is an identifier? When can they be finalized?
One or more attributes can serve as the entityidentifier, uniquely identifying each entity instance Concatenated identifier consists of several attributes An identifier may be 'artificial,' such as creating an ID number Final decision on identifiers may postponed to the Design Phase
What is modality?
Refers to whether or not an instance of a child entity can exist without a related instance in the parent entity o Not Null - an instance in the related entity must exist for an instance in another entity to be valid o Null - no instance in the related entity is necessary for an instance in another entity to be valid
How do you Draw the Relationships? (Step 3)
Start with an entity and identify all entities with which it shares relationships Describe the relationship with the appropriate verb phrase Determine the cardinality (1:1, 1:N, M:N) and modality (null, not null) by discussing the business rules with knowledgeable users Dependent or Independent?
What is balancing ERDs with DFDs?
The DFD data components need to balance the ERD's data stores (entities) and data elements (attributes) Check that all data stores and elements correspond between models o Data that is not used is unnecessary o Data that has been omitted results in an incomplete system
What is a deletion anomaly? How to resolve?
When attempting to remove certain information from an instance of an entity ( particular student drops a course ), you end up losing other important information in doing so. If the course, and student are in the same entity, we may lose student information when dropping a course. Normalize and separate the course entity from student entity.
What is a data model and what does it do?
o A formal way of representing the data that are used and created by a business system o Shows the people, places and things about which data is captured and the relationships among them. o Logical data model shows the organization of data without indicating how it is stored, created, or manipulated o Physical data model shows how the data will actually be stored in databases or files.
Ways to identify Attributes?
o Attributes are "properties" of an entity that are used by at least one business process. Broken done into its most useful level of detail. (e.g. DOB) o Check the process model repository entries for details on data flows and data stores o Check the data requirements of the requirements definition o Interview knowledgeable users o Perform document analysis on existing forms and reports
What type of relationship is typically "problematic"? What should be done?
o M:N relationships are "problematic" o Remove the M:N relationship between two entities and insert new entity between them o Create two 1:N relationships: original entities are parents to the new child intersection entity o Name the intersection entity o Migrate parent entity primary keys to new entity as foreign keys (possibly also concatenated primary key)
What are the steps to build an ERD?
o Step 1: Identify the entities o Step 2: Add appropriate attributes for each entity o Step 3: Draw the relationships that connect associated entities