Chapter 3 - Relational Database Model

SELECT

(1) A SQL command that yields all the values of all rows or a subset of rows in a table. The SELECT statement is used to retrieve data from tables. (2) In relational algebra, an operator to select a subset of rows. Also known as RESTRICT.

Data Dictionary

A DBMS component that stores metadata- data about data. Thus, the ____________________ _____________ contains the data definition as well as their characteristics and relationships. It may also include data that are external to the DBMS.

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 sysem data dictionary that describes all objects in a database.

Inner Join

A join operation in which only rows that meet a given criterion are selected. The join criterion can be an equality condition (natural or equijoin) or an inequality condition (theta join). The _________________ ________________ is the most commonly used type of join. Contract with Outer Join.

Equijoin

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

Theta Join

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

Secondary Key

A key strictly for data retrieval purposes. For example, customer 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. See also, Key.

Candidate Key

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

Composite Key

A multiple-attribute key is known as a ____________ ___________.

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.

Closure

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

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.

Natural Join

A relational operation that yields a new table composed of only the rows with common values in their common attributes.

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.

Superkey

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

Dependent

An attribute whose value is determined by another attribute is known as a ______________.

Composite Entity

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

Unique Index

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

Index

An ordered array of key values and row ID values (pointers). An ____________ is 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. See also Boyce-Codd Normal Form (BCNF)

Key Attribute

Attributes that form a primary key. See also, prime attribute.

Join Column(s)

Columns that are used in the criteria of join operations. The ______________ ________________ generally share similar values (have a compatible domain).

Associative, bridge

Composite Entity is synonymous with an ________________ entity and a ____________ entity.

Right Outer Join

In a pair of table to be joined, a join that yields all of the rows in the right table, including 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 h have a matching CUSTOMER row. See also Left Outer Join and Outer Join.

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 with AGENT will yield all of the CUSTOMER rows, including he ones that do not have matching AGENT row. See also Outer Join and Right Outer Join.

Domain

In data modeling, the construct used to organize and describe an attributes set of possible values is the ________________.

JOIN

In relational algebra, a type of operator used to yield rows from two tables based on criteria. There are many types of joins, such as natural join, theta join, equijoin, and outer join.

DIFFERENCE

In relational algebra, an operation used to yield all rows from one table that are not found in another union-compatible table.

DIVIDE

In relational algebra, an operator that answers queries about one set of data being associated with all values of data in another set of data.

UNION

In relational algebra, an operator used to merge (append) two tables into a new table, dropping the duplicate rows. The tables must be union-compatible.

PROJECT

In relational algebra, an operator used to select a subset of columns.

PRODUCT

In relational algebra, an operator used to yield all possible paris of rows from two tables. Also known as the cartesian product.

INTERSECT

In relational algebra, an operator used to yield only the rows that are common to two union compatible tables.

Linking Table

In the relational model, a table that implements an M:M relationship. See also Composite Entity.

Primary Key

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

Key

One or more attributes that determine other attributes. See also superkey, candidate key, primary key, secondary key, and foreign key.

RESTRICT

See SELECT.

Relvar

Short for relation variable, a variable that holds a relation. A relvar is a container (variable) for holding relation data, not the relation itself. )

Flags

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

Attribute Domain

The ______________ _______________is the set of values allowed in an attribute.

Null

The absence of an attribute value. Note that this is not a blank.

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 is known as _______________ ________________-.

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 value of attribute B can be looked up.

Synonym

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

Homonym

The use of the same name to label different attributes. ________________ generally should be avoided. Some relational software automatically checks for these and either alerts the user to their existence or automatically makes the appropriate adjustments. See also, Synonym.

Union-Compatible

Two or more tables that have the same number of columns and the corresponding columns have compatible 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.

Functional Dependent

Within a relation R, and 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 as A B

Foreign Key

________________ ____________ is an attribute or attributes in one table whose values must match the primary key in another table or whose values must be null. See key.