Chapter 2 Database Management – Flashcards
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Business rules are important in data modeling because they...
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Govern how data are handled and stored, and govern the people, places, events, processes, networks, and objectives of the organization
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Examples of basic business rule are (2)
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data names and definitions, constraints on data objects
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The ER model is most used as a
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tool for communications between database designers and end users during the ANALYSIS phase of database development
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Entity-relationship model (E-R model)
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a detailed, logical representation of the data for an organization or for a business area.
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E-R model mostly expressed as a
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Entity-relationship diagram
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Business rules and policies are NOT
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universal.
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The purpose of data modeling is to
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document business rules.
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Business Rule (definition)
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a statement that defines or constrains some aspect of the business.
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A business rule is intended to...
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assert business structure or to control or influence the behavior of the business.
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Business rules are expressed in terms
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familiar to end users
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Business rules are automated through
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DBMS software
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Business rule are highly
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maintainable, stored in central repository, each rule expressed only once.
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Good business rules are...(7)
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1. declarative 2. precise 3. atomic 4. consistent 5. expressible 6. distinct 7. business-oriented
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Declarative
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it is a statement of the policy, it is what not HOW
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Precise
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the rule must have only one agreed-upon meaning and interpretation
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Atomic
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A business rule marks one statement, not several.
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Consistent
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Business Rule must be internally consistent and externally consistent and can not contradict other rules
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Expressible
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be stated in natural, structured language
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Distinct
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Business rules are not redundant
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Business-Oriented
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stated in terms that business people can understand
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Data objects must be ____ before
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named, before they can be used unambiguously in a model of organizational data
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A good data name is (7)
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1. Related to business 2. meaningful and self-documenting 3. Unique 4. Readable 5. Composed of words from an approved list 6. Repeatable 7. Written in standard syntax
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Data Definitions
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Explanation of a term or fact.
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Term
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word or phrase that has specific meaning for the business. Key words used to form data names. ex:// course, section, rental car, flight.
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No need to define common terms like person, month, television, because
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there terms are understood without ambiguity by most persons.
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Fact
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is an association between two or more terms. Documented as a simple declarative statement that relates terms.
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Guidelines for good data definition (4)
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1. Gathered in conjunction with system requirements 2. Accompanied by diagrams 3. Concise description of essential data meaning 4. Achieved by consensus, and iteratively refined
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Entities
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a person, place, object, event, or a concept in the under environment about which the organization wishes to maintain data. Entity has a noun name.
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Entities often correspond to a
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row in a table
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Entity type is ALWAYS
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singular
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Entity Type
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a collection of entities that share common properties or characteristics. (often corresponds to a table). use capital letters for names of entity types.
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Entity Instance
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a single occurrence of an entity type. ex:// there may be one EMPLOYEE entity type in most organizations, but there may be hundreds of instances of this entity type stored in the database.
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If an entity is the result of extracting data from the database,
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don't represent as it's own entity (ex://= figure 2-4)
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Strong Entity Type
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is one that exists independently of other entity types. ex:// STUDENT, EMPLOYEE, AUTOMOBILE, COURSE
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Instances of a strong entity type always have
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a unique characteristic (an identifier)
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Weak Entity Type
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an entity whose existence depends on some other entity type. Has no business meaning in an E-R diagram without the entity on which it depends. Does not have its own identifier.
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Typically a weak entity type has an
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attribute that services as a partial identifier.
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Partial Identifier is indicated with a
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double underline
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Identifying owner
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The entity type on which the weak entity type depends.
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Identifying relationship
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the relationship between a weak entity type and its owner.
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Attribute
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a property or characteristic of an entity type that is of interest to the organization
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Required Attribute & ex
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an attribute that must have a value for every entity (or relationship) instance with which it is associated. Ex:// student ID or student address (boldface, or notified with symbol)
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Optional Attribute
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An attribute that may not have a value for every entity (or relationship) instance with which it is associated. (notified by normal font)
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An attribute wihtout a value is said to be
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nul
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Composite Attribute, how to indicate, example
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An attribute that has meaningful component parts that can be broken down. (Ex:// employee address, employee name) Use parenthesis to indicate.
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Simple Attribute
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an attribute that cannot be broken down into smaller components that are meaningful to the organization. Ex:// Vehicle ID
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Multivalued Attribute, Example, how indicate
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An attribute that may take on more than on value for a given entity (or relationship) instance. Use curly brackets to indicate. Ex:/ skill
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Difference between multivalued and composite attribute
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A multivalued attribute may occur one or multiple times, a composite attribute each has same number of components.
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Derived Attribute, how to indicate
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an attribute whose values can be calculated from related attribute values. Indicate by using square brackets around attribute name. Ex:// want to know how many years employed, use Date Employed and today's date to find column value.
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Identifier, how to indicate
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An attribute whose value distinguishes instances of an entity type. Underline to indicate.
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Composite Identifier
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An identifier that consists of a composite attribute. Ex:// Flight ID needs Flight Number, and Date) composite attributes to uniquely identify the entity.
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What should entity be? (3)
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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.
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What should an entity NOT be? (2)
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-A user of the database system, - an output of the database system (report)
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Candidate Key
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an attribute that could be a key.. satisfies the requirements of being a key
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Criteria for selecting identifiers(4)
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1. Will not change in value over the life of each instance of the entity type 2. Will not be NULL 3. No intelligent Identifiers (containing locations or people that might change) 4. Substitute new, simple keys for long, composite keys
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Relationship, what type of name
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is an association representing an interaction among the instances of one or more entity type that is of interest to the organization. Has a verb phrase name**. Relationships and cardinalities represent business rules.
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Relationship Type
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a meaningful association between (or among) entity types. Implies that the relationship allows us to answer questions that could not be answered given only the entity types. Denoted with line and labeled with the name of the relationship. ex:// completes, calls, hires etc.
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Relationship Instance
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an association between or among entity instances, where each relationship instance associates exactly one entity instance from each participating entity type.
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Relationships can have attributes that describe
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features pertaining to the association between the entities in the relationship.
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Two entities can have
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more than one type of relationship between them.
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Associative Entity
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combination of relationship and entity. ** the relationship name is usually converted to an entity name that is a noun. No relationship on lines between association because the associative entity represents the relationship. EX:// certificate
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Four conditions to concert a relationship to an associative entity type:
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1. All relationships for participating entity types are "many" 2. the resulting associative entity type has independent meaning to end users and (preferably) can be identified with a single-attribute identifier. 3. The associative entity has one or more attributes in addition to the identifier. 4. The associative entity participates in one or more relationships independent of the entities related in the associated relationship.
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Degree of a relationship
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the number of entity types that participate in a relationship.
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Three most common relationship degrees:
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1. Unary (degree 1) 2. Binary (degree 2) 3. Ternary (degree 3)
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Unary Relationship
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a relationship between the instances of a single entity type.
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Binary Relationship
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a relationship between the instances of two entity types and is the most common type of relationship encountered in data modeling.
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Ternary Relationship
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simultaneous relationship among the instances of three entity types.
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Guideline for Ternary relationships
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convert all ternary (or higher) relationships to associative entities.
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When should an attribute be linked to an entity type via a relationship?
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When the attribute is the identifier or some other characteristic of an entity type in the data model and multiple entity instances need to share thee same attributes.
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Cardinality constriant
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specifies that the number of instances of entity B that can (or must) be associated with each instance of entity A.
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Minimum Cardinality
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minimum number of instances of entity B that may be associated with each instance of entity A. if minimum is one, participation is mandatory. If minimum is zero, participation is optional.
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Maximum Cardinality
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the maximum number of instances of entity B that may be associated with each instance of entity A.
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Time Stamp
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simply a time value, such as date and time, that is associated with a data value. A time stamp may be associated with any data value that changes over time when we need to maintain a history of those data values.
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Time stamps may be recorded when:
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1. indicate the time the value was entered 2. the time the value becomes valid or stops being valid 3. time when critical actions were performed, such as updates, corrections, or audits.
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Guidelines for naming relationships
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1. Relationship name is verb phrase 2. avoid vague names
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Guidelines for defining relationships:
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1. Relationship definition explains what action is being taken a possibly why it is important. Not important to explain how the action is taken. 2. Relationship should explain optional participation,
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Levels of Database Schemas
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1. External level; combines external vews 2. Conceptual Schema 3. Internal Schema
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Whenever have MULTIVALUED attributes or COMPOSITE ATTRIBUTES =
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they can be turned into associative entities. Turn composite attributes into new entities!!!
