Database Systems Chapter 3
Data dictionary
A DBMS component that stores metadata—data about data. Thus, it contains the data definition as well as their characteristics and relationships. May also include data that are external to the DBMS. Also known as an information resource dictionary.
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 operation 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). 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.
Theta join
A join operator that links tables using an inequality comparison operator (<, >, <=, >=) in the join condition.
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
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. Contrast with inner join. See also left outer join and right outer join.
Natural join
A relational operation that yields a new table composed of only the rows with common values in their common attribute(s).
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.
Foreign key (FK)
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.
Superkey
An attribute or attributes that uniquely identify each entity in a table.
Dependent
An attribute whose value is determined by another attribute.
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. Also known as a bridge entity.
Key
An entity identifier based on the concept of functional dependence; may be classified in serval ways
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). Are generally used to speed up and facilitate data retrieval. Also known as an index key.
Determinant
Any attribute in a specific row whose value directly determines other values in that row.
Join column(s)
Columns that join two tables. Generally share similar values.
Null
In SQL, the absence of an attribute value. Note that a null is not a blank.
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.
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
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 (PK)
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 entity identifier.
Secondary key
Key used strictly for data retrieval purposes
Associative entity
Related to composite entity
Attribute domain
Related to domain
Relvar
Short for relation variable, a variable that holds a relation. 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. 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.
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 value of attribute B can be looked up.
Synonym
The use of different names to identify the same object, such as an entity, an attribute, or an relationship
Homonym
The use of the same name to label different attributes. 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.
Functional dependence
Within a relation R, an attribute B is functionally 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.
