Database Systems, Chapter 2-3 Definitions

Theta Join

A join operator that links tables using an inequality comparison operator (<,>,=,>=, <=) in the join condition.

Composite Key

A multiple attribute key.

Regular Entity

An entity that can exist apart from all of its related entities. Existence Independent.

Unique Index

An index in which the index key can have only one associated pointer value (row).

index

An ordered array of index key values and row ID values (pointers). Indexes are generally used to speed up and facilitate data retrieval.

Synonym

The use of different names to identify the same object, such as an entity, an attribute, or a relationship; synonyms should generally be avoided.

Functional Dependence

Within a relation R, an attribute B is functionally dependent on an attribute A if and only if a given value of attribute A determines exactly one value of attribute B. The relationship "B is dependent on A" is equivalent to "A determines B," and is written AB.

Data Dictionary

A DBMS component that stores metadata -- data about data. Thus, the data dictionary contains the data definition as well as their characteristics and relationships. A data dictionary may also include data that are external to the DBMS. Also known as the information resource directory.

Referential Integrity

A condition by which a dependent table's foreign key must have either a null entry or a matching entry in the related table. Even though an attribute may not have a corresponding attribute, it is impossible to have an invalid entry.

Full Functional Dependence

A condition in which an attribute is functionally dependent on a composite key but not on any subset of the key.

System Catalog

A detailed system data dictionary that describes all objects in a database.

Inner Join

A join operations in which only rows that meet a given criterion are selected. The join criterion can be an equality condition (natural join or equijoin) or an inequality condition (theta join). The inner join is the most commonly used type of join. Contrast with outer join.

Equijoin

A join operator that links tables based on an equality condition that compares specified columns of the tables.

Secondary Key

A key used strictly for data retrieval purposes. For Example, Customers are not likely to know their customer number(primary key), but the combination of last name, first name, middle initial, and telephone number will probably match the appropriate table row.

Candidate Key

A minimal superkey; that is, a key that does not contain a subset of attributes that is itself a superkey.

Set Theory

A part of mathematical science that deals with sets, or groups of things, and is used as the basis for data manipulation in the relational model.

Iterative process

A process based on repetition of steps and procedures.

Existence-Independent

A property of an entity that can exist apart from one or more related entities. Such a table must be created first when referencing an existence-dependent table.

existence-dependent

A property of an entity whose existence depends on one or more other entities. In such an environment, the existence-independent table must be created and loaded first because the existence-dependent key cannot reference a table that does not yet exist.

Closure

A property of relational operators that permits the use of relational algebra operators on existing tables (relations) to produce new relations.

Cardinality

A property that assigns a specific value to connectivity and expresses the range of allowed entity occurrences associated with a single occurrence of the related entity.

Outer Join

A relational algebra JOIN operation that produces a table in which all unmatched pairs are retained; unmatched values in the related table are left null. Contrast with inner join.

Natural Join

A relational operation that links tables by selecting only the rows with common values in their common attribute(s).

Recursive Relationships

A relationship found within a single entity type. For example, an EMPLOYEE is married to an EMPLOYEE or a PART is a component of another PART.

Mandatory participation

A relationship in which one entity occurrence must have a corresponding occurrence in another entity. For example, an EMPLOYEE works in a DIVISION. ( A person cannot be an employee without being assigned to a company's division.)

Identifying relationship

A relationship in which 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 of the parent entity.

Non-identifying relationships

A relationship in which the primary key of the dependent ( many side) entity does not contain the primary key of the related parent entity. Also known as a weak relationship.

Weak Relationship

A relationship in which the primary key of the related entity does not contain a primary key component of the parent entity. Also known as a non-identifying relationship.

Strong Relationship

A relationship that occurs when two entities are existence-dependent; from a database design perspective, this relationship exists whenever the primary key of the related entity contains the primary key of the parent entity.

Relational Algebra

A set of mathematical principles that form the basis for manipulating relational table contents; the eight main functions are SELECT, PROJECT, JOIN, INTERSECT, UNION, DIFFERENCE, PRODUCT, and DIVIDE.

Binary Relationship

An ER term for an association (relationship) between two entities. For example, PROFESSOR teaches COURSE.

Participants

An ER term for entities that participate in a relationship. For example, in the relationship "PROFESSOR teaches CLASS," the teaches relationship is based on the participants PROFESSOR and CLASS.

Ternary Relationship

An ER term used to describe an association (relationship) between three entities. For example, a CONTRIBUTOR contributes money to a FUND from which a RECIPIENT receives money.

Unary Relationship

An ER term used to describe an association within an entity. For example, a COURSE might be a prerequisite to another COURSE.

Foreign Key

An attribute of attributes in one table whose values must match the primary key in another table or whose values must be null.

Superkey

An attribute or attributes that uniquely identify each entity in a table.

Composite Attribute

An attribute that can be further subdivided to yield additional attributes. For example, a phone number such as 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

Multivalued attributes

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.

Single-valued attribute

An attribute that can have only one value.

Simple Attribute

An attribute that cannot be subdivided into meaningful components. Compare to composite attribute.

Derived attribute

An attribute that does not physically exist within the entity and is derived via an algorithm. For example, the Age attribute might be derived by subtracting the birth date from the current date.

Dependent

An attribute whose value is determined by another attribute.

Bridge Entity

An entity designed to transform an M:N relationship into two 1:M relationships. The composite entity's primary key comprises at least the primary keys of the entities that it connects.

Composite Entity

An entity designed to transform an M:N relationship into two 1:M relationships. The composite entity's primary key comprises at least the primary keys of the entities that it connects.

Associative Entity

An entity designed to transform an M:N relationship into two 1:M relationships. The composite entity's primary key comprises at least the primary keys of the entities that it connects. Also known as bridge entity.

key

An entity identifier based on the concept of functional dependence; keys may be classified in several ways.

Strong entity

An entity that can exist apart from all of its related entities Existence Independent.

Weak Entity

An entity that displays existence dependence and inherits the primary key of its parent entity. For example, a DEPENDENT requires the existence of an EMPLOYEE.

index key

An ordered array of index key values and row ID values (pointers). Indexes are generally used to speed up and facilitate data retrieval.

Determinant

Any attribute in a specific row whose value directly determines other values in that row.

Join Columns

Columns that join two tables. The join columns generally share similar values.

Optional participation

In ER Modeling, a condition in which one entity occurrence does not require a corresponding entity occurrence in a particular relationship.

Composite Identifier

In ER modeling, a key composed of mare than one attribute.

optional attribute

In ER modeling, an attribute that does not require a value; therefore, it can be left empty.

Required Attribute

In ER modeling, an attribute that must have a value. In other words, it cannot be left empty.

Null

In SQL, the absence of an attribute value. Note that a null is not a blank.

Left Outer Join

In a pair of tables to be joined a join that yields all the rows in the left table, including those that have no matching values in the other table. For example, a left outer join of CUSTOMER and AGENT will yield all of the CUSTOMER rows, including the ones that do not have a matching AGENT row.

Right Outer Join

In a pair of tables to be joined, a join that yields all of the rows in the right table, including the ones with no matching values in the other table. For example a right outer join of CUSTOMER with AGENT will yield all of the AGENT rows, including the ones that do not have a matching customer row.

Identifiers

In an ERM, unique names of each entity instance. In the relational model, such identifiers are mapped to the primary keys in tables.

Attribute Domain

In data modeling, the construct used to organize and describe an attribute's set of possible values.

Domain

In data modeling, the construct used to organize and describe an attribute's set of possible values.

Tuple

In the relational model, a table row.

Linking Table

In the relational model, a table that implements an M:M relationship.

Primary Key

In the relational model, an identifier composed of one or more attributes that uniquely identifies a row. Also, a candidate key selected as a unique identifier.

Flags

Special codes implemented by designers to trigger a required response, alert end users to specified conditions, or encode values. Flags may be used to prevent nulls by bringing attention to the absence of a value in a table.

Key Attribute

The attributes that form a primary key.

Connectivity

The classification of the relationship between entities. Classifications include 1:1, 1:M, and M:N.

Relationship Degree

The number of entities or participants associated with a relationship. A relationship degree can be unary, binary, ternary, or higher.

Relational Schema

The organization of a relational database as described by the database administrator.

Entity Integrity

The property of a relational table that guarantees each entity has a unique value in a primary key and that the key has no null values.

Determination

The role of a key. In the context of a database table, the statement "A determines B" indicates that knowing the value of attribute A means that the vale of attribute B can be looked up.

Homonym

The use of he same name to label different attributes. Homonyms generally should be avoided. Some relational software automatically checks for homonyms and either alerts the user to their existence or automatically makes the appropriate adjustments.

Union Compatible

Two or more tables that share the same column names and have columns with compatible data types or domains.

Predicate Logic

Used extensively in mathematics to provide a framework in which an assertion (statement of fact) can be verified as either true or false.