Term
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Definition
| An ER term used to describe an association (relationship) between two entities. Example: PROFESSOR teaches COURSE |
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Term
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Definition
| Assigns a specific value to connectivity. Expresses the range (minimum to maximum) of allowed entity occurrences associated with a single occurrence ofthe related entity. |
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Term
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Definition
| An attribute that can be further subdivided to yield additional attributes. For example, a phone number (615-898-2368) may be divided into an area code (615), an exchange number (898), and a four-digit code (2368). Compare to simple attribute. |
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Term
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Definition
| In ER modeling, a key composed of more than one attribute. |
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Term
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Definition
| Describes the classification ofthe relationship between entities. Classifications include 1:1, 1:M, and M:N. |
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Term
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Definition
| An attribute that does not physically exist within the entity and is derived via an algorithm. Example: Age = current date birth date. |
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Term
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Definition
| A property of an entity whose existence depends on one or more other entities. In an existence-dependent environment, the existenceindependent table must be created and loaded first because the existencedependent key cannot reference a table that does not yet exist. |
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Term
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Definition
| An entity that can exist apart from one or more related entities. It must be created first when referencing an existence-dependent table to it |
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Term
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Definition
| The ERM uses identifiers to uniquely identify each entity instance. In the relational model, such identifiers are mapped to prin1ary keys in tables |
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Term
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Definition
| A relationship that exists when the related entities are existence-dependent. Also called a strong relationship or strong identifying relationship because the dependent entity's primary key contains the primary key ofthe parent entity |
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Term
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Definition
| A process based on repetition of steps and procedures |
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Term
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Definition
| A term used to describe a relationship in which one entity occurrence must have a corresponding occurrence in another entity. Example: EMPLOYEE works in DIVISION. (A person cannot be an employee if he or she is not assigned to a company's division.) |
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Term
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Definition
| An attribute that can have many values for a single entity occurrence. For example, an EMP _DEGREE attribute might store the string "BBA, MBA, PHD" to indicate three different degrees held. |
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Term
| non-identifying relationship |
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Definition
| A relationship that occurs when the primary key ofthe dependent (many side) entity does not contain the primary key of the related parent entity. Also known as a weak relationship. |
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Term
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Definition
| In ER modeling, refers to an attribute that does not require a value, therefore it can be left empty. |
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Term
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Definition
| In ER modeling, refers to a condition where one entity occurrence does not require a corresponding entity occurrence in a particular relationship. |
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Term
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Definition
| An ER term used to label the entities that participate in a relationship. Example: PROFESSOR teaches CLASS. (The teaches relationship is based on the participants PROFESSOR and CLASS.) |
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Term
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Definition
| A relationship that is found within a single entity type. For example, an EMPLOYEE is married to an EMPLOYEE or a PART is a component of another PART. |
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Term
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Definition
| ndicates the number of entities or participants associated with a relationship. A relationship degree can be unary, binary, ternary, or higher level. |
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Term
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Definition
| In ER modeling, refers to an attribute that must have a value. In other words, it cannot be left empty. |
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Term
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Definition
| An attribute that cannot be subdivided into meaningful components. Compare to composite attribute. |
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Term
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Definition
| An attribute that can have only one value. |
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Term
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Definition
| When two entities are existence-dependent; from a database design perspective, this exists whenever the primary key ofthe related entity contains the primary key ofthe parent entity. |
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Term
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Definition
| An ER ternl used to describe an association (relationship) between three entities. Example: A CONTRIBUTOR contributes money to a FUND from which a RECIPIENT receives money. |
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Term
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Definition
| An ER term used to describe an association within an entity. Example: A COlTRSE is a prerequisite to another COURSE. |
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Term
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Definition
| An entity that displays existence dependence and inherits the primary key of its parent entity. Example: A DEPENDENT requires the existence of an EMPLOYEE. |
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Term
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Definition
| A relationship that exists when the PK ofthe related entity does not contain a PK component of the parent entity. Also known as a nonidentifying relationship. |
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Term
| ERD Entity Relationship Model 1 of three components |
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Definition
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Term
| ERD Entity Relationship Model 1 of three components |
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Definition
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Term
| ERD Entity Relationship Model 1 of three components |
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Definition
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Term
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Definition
Refers to entity set and not to single entity occurrence
Corresponds to table and not to row in relational environment
In Chen and Crow’s Foot models, entity represented by rectangle with entity’s name
Entity name, a noun, written in capital letters |
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Term
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Definition
Characteristics of entities
Chen notation: attributes represented by ovals connected to entity rectangle with a line
Each oval contains the name of attribute it represents
Crow’s Foot notation: attributes written in attribute box below entity rectangle |
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Term
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Definition
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Term
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Definition
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Term
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Definition
| set of possible values for an attribute |
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Term
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Definition
| one or more attributes that uniquely identify each entity instance |
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Term
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Definition
| primary key composed of more than one attribute |
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Term
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Definition
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Term
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Definition
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Term
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Definition
| can have only a single value |
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Term
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Definition
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Term
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Definition
| value may be calculated from other attributes |
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Term
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Definition
| need not be physically stored within database |
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Term
| Relationships that should not be implented |
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Definition
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Term
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Definition
| entities that participate in a relationship |
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Term
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Definition
| Association between entities |
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Term
| Relationships between entities |
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Definition
| always operate in both directions |
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Term
| Relationship classification is difficult to establish |
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Definition
| if only one side of the relationship is known |
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Term
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Definition
| Describes the relationship classification |
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Term
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Definition
| Expresses minimum and maximum number of entity occurrences associated with one occurrence of related entity |
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Term
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Definition
| Entity exists in database only when it is associated with another related entity occurrence |
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Term
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Definition
Entity can exist apart from one or more related entities
Sometimes such an entity is referred to as a strong or regular entity |
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Term
| Weak (non-identifying) relationships |
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Definition
| Exists if PK of related entity does not contain PK component of parent entity |
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Term
| Strong (identifying) relationships |
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Definition
| Exists when PK of related entity contains PK component of parent entity |
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Term
| Weak entity meets two conditions |
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Definition
Existence-dependent
Primary key partially or totally derived from parent entity in relationship |
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Term
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Definition
| One entity occurrence does not require corresponding entity occurrence in particular relationship |
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Term
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Definition
| One entity occurrence requires corresponding entity occurrence in particular relationship |
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Term
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Definition
| Indicates number of entities or participants associated with a relationship |
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Term
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Definition
| Association is maintained within single entity |
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Term
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Definition
| Two entities are associated |
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Term
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Definition
| Three entities are associated |
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Term
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Definition
Relationship can exist between occurrences of the same entity set
Naturally found within unary relationship |
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Term
| Associative (Composite) Entities |
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Definition
| Also known as bridge entities |
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Term
| Associative (Composite) Entities |
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Definition
| Used to implement M:N relationships |
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Term
| Associative (Composite) Entities |
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Definition
| Composed of primary keys of each of the entities to be connected |
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Term
| Associative (Composite) Entities |
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Definition
| May also contain additional attributes that play no role in connective process |
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Term
| Database Design Challenges: Conflicting Goals |
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Definition
| design standards, processing speed, information requirements |
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Term
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Definition
Entities
Relationships
Attributes
Includes connectivity and cardinality notations |
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Term
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Definition
Generic entity type related to one or more entity subtypes
Contains common characteristics |
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Term
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Definition
| Contains unique characteristics of each entity subtype |
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Term
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Definition
Also known as non-overlapping subtypes
Subtypes that contain unique subset of supertype entity set |
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Term
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Definition
| Subtypes that contain nonunique subsets of supertype entity set |
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Term
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Definition
Symbolized by a circle over a single line
Some supertype occurrences that are not members of any subtype |
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Term
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Definition
Symbolized by a circle over a double line
Every supertype occurrence must be member of at least one subtype |
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Term
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Definition
Identifies more specific entity subtypes from higher-level entity supertype
Top-down process
Based on grouping unique characteristics and relationships of the subtypes |
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Term
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Definition
Identifies more generic entity supertype from lower-level entity subtypes
Bottom-up process
Based on grouping common characteristics and relationships of the subtypes |
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Term
| Entity Integrity: Selecting Primary Keys |
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Definition
Primary key most important characteristic of an entity
Single attribute or some combination of attributes |
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Term
| Entity Integrity: Selecting Primary Keys |
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Definition
| Primary key’s function is to guarantee entity integrity |
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Term
| Entity Integrity: Selecting Primary Keys |
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Definition
| Primary keys and foreign keys work together to implement relationships |
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Term
| Entity Integrity: Selecting Primary Keys |
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Definition
| Properly selecting primary key has direct bearing on efficiency and effectiveness |
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Term
| Natural Keys and Primary Keys |
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Definition
Natural key is a real-world identifier used to uniquely identify real-world objects
Familiar to end users and forms part of their day-to-day business vocabulary
Generally data modeler uses natural identifier as primary key of entity being modeled
May instead use composite primary key or surrogate key |
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Term
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Definition
Attribute that uniquely identifies entity instances in an entity set
Could also be combination of attributes
Main function is to uniquely identify an entity instance or row within a table
Guarantee entity integrity, not to “describe” the entity
Primary keys and foreign keys implement relationships among entities
Behind the scenes, hidden from user |
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Term
| Primary Key Characteristic: Unique values |
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Definition
The PK must uniquely identify each entity instance. A primary key must be able
to guarantee unique values. It cannot contain nulls |
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Term
| Primary Key Characteristic: nonintelligent |
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Definition
The PK should not have embedded semantic meaning (factless). An attribute
with embedded semantic meaning is probably better used as a descriptive char-
acteristic of the entity rather than as an identifier. In other words, a student ID of
650973 would be preferred over Smith, Martha L. as a primary key identifier. In
short, the PK should be factless. |
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Term
| Primary Key Characteristic: No Change Over time |
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Definition
If an attribute has semantic meaning, it might be subject to updates. This is why
names do not make good primary keys. If you have Vickie Smith as the primary
key, what happens when she gets married? If a primary key is subject to change,
the foreign key values must be updated, thus adding to the database work load.
Furthermore, changing a primary key value means that you are basically chang-
ing the identity of an entity. In short, the PK should be permanent and
unchangeable. |
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Term
| Primary key characterisic preferable single-attribute |
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Definition
A primary key should have the minimum number of attributes possible
(irreducible). Single-attribute primary keys are desirable but not required. Single-
attribute primary keys simplify the implementation of foreign keys. Having
multiple-attribute primary keys can cause primary keys of related entities to
grow through the possible addition of many attributes, thus adding to the data-
base work load and making (application) coding more cumbersome. |
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Term
| Primary key charactrisic preferable numeric |
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Definition
Unique values can be better managed when they are numeric, because the
database can use internal routines to implement a counter-style attribute that
automatically increments values with the addition of each new row. In fact, most
database systems include the ability to use special constructs, such as Autonum-
ber in Microsoft Access, to support self-incrementing primary key attributes |
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Term
| When to Use Composite Primary Keys |
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Definition
As identifiers of composite entities
Where each primary key combination allowed once in M:N relationship
As identifiers of weak entities
Where weak entity has a strong identifying relationship with the parent entity
Automatically provides benefit of ensuring that there cannot be duplicate values |
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Term
| When to Use Composite Primary Keys |
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Definition
When used as identifiers of weak entities normally used to represent:
Real-world object that is existent-dependent on another real-world object
Real-world object that is represented in data model as two separate entities in strong identifying relationship |
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Term
| When To Use Surrogate Primary Keys |
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Definition
No natural key
Selected candidate key has embedded semantic contents
Selected candidate key is too long or cumbersome |
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Term
| When To Use Surrogate Primary Keys |
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Definition
Ensure that candidate key of entity in question performs properly
Use “unique index” and “not null” constraints |
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Term
| Implementing 1:1 Relationships |
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Definition
Foreign keys work with primary keys to properly implement relationships in relational model
Put primary key of the “one” side on the “many” side as foreign key
Primary key: parent entity
Foreign key: dependent entity |
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Term
| Implementing 1:1 Relationships |
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Definition
Place a foreign key in both entities (not recommended)
Place a foreign key in one of the entities
Primary key of one of the two entities appears as foreign key of other |
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Term
| Maintaining History of Time-Variant Data |
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Definition
Normally, existing attribute values replaced with new value without regard to previous value
Time-variant data:
Values change over time
Must keep a history of data changes
Keeping history of time-variant data equivalent to having a multivalued attribute in your entity
Must create new entity in 1:M relationships with original entity
New entity contains new value, date of change |
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Term
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Definition
occurs when relationship is improperly or incompletely identified
Represented in a way not consistent with the real world |
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Term
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Definition
Occur when there are multiple relationship paths between related entities
Main concern is that redundant relationships remain consistent across model
Some designs use redundant relationships to simplify the design |
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