Chapter 3 - Relational Database Model

associative entity

(Aka bridge entity or composite entity) An entity designed to transform a 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. (Linking table is the implementation of an associative entity/composite entity/bridge entity).

bridge entity

(Aka bridge entity or composite entity) An entity designed to transform a 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. (Linking table is the implementation of an associative entity/composite entity/bridge entity).

composite entity

(Aka bridge entity or composite entity) An entity designed to transform a 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. (Linking table is the implementation of an associative entity/composite entity/bridge entity).

data dictionary

(Aka information resource dictionary)A DBMS component that stores metadata (data about data). Thus, the data dictionary contains the data definitions as well as its characteristics and relationships. A data dictionary may also include data that are external to the DBMS. Also known as an information resource dictionary.

primary key(PK)

A candidate key selected as a unique entity identifier.

referential integrity

A condition in which a dependent table's foreign key must have either a null entry our 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 that composite key.

system catalog

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

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 that is used strictly for data retrieval purposes.

set theory

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

candidate key

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

composite key

A multiple-attribute key.

closure

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

Natural join

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

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.

relational algebra

A set of mathematical principles that form the basis of the manipulation of relational table contents; composed of eight main functions: SELECT, PROJECT, JOIN, INTERSECT, UNION, DIFFERENCE, and DIVIDE.

Join column(s)

A term used to refer to the columns that join two tables. The join columns generally share similar values.

foreign key(FK)

An attribute(or combination 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 combination of attributes) that uniquely identifies each entity in a table.

Key

An entity identifier based on the concept of functional dependence; may be classified as follows: superkey, candidate key, primary key, secondary key, and foreign key.

unique index

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

Index

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

Index key

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

determination

Any attribute in a specific row whose value directly determines other values in that row. (The key's role is based on this concept).

Null

In SQL, refers to the absence of an attribute value. Note: A null is not blank.

Left outer join

In a pair of tables to be joined, a left outer join yields all of 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 the ones that do not have matching Agent row.

right outer join

In a pair of tables to be joined, a right outer join 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.

tuple

In a relational model, a table row.

Linking table

In a relational model, a table that uses an associative entity/composite entity/bridge entity in place of a M:M relationship. It contains at least the foreign keys of the linked tables.

attribute domain

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

domain

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

Homonyms

Indicates the use of the same name to label different attributes; generally should be avoided. Some relational software automatically checks for homo-nyms and either alerts the user to their existence or automatically makes the appropriate adjustments.

flags

Special codes implemented by designers to trigger a required response, to 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 attribute(s) that form(s) a primary key.

relational schema

The description of the organization of a relational database as seen by the database administrator.

entity integrity

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

synonym

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

union-compatible

Two or more tables are union-compatible when they share the same column names and the columns have compatible data types or domains.

Predicate logic

Used extensively in mathematics, provides a framework in which an assertion (statement of fact) can be verified as either true or false. For example, suppose that a student with student ID of 12345678 is named Melissa Sanduski. The assertion can be easily be demonstrated to be true or false.

Functional dependence

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