C192 - Chapter 04
Relational database
A collection of normalized relations with distinct relation names.
Superkey
An attribute, or set of attributes, that uniquely identifies a tuple within a relation. However, a _____ may contain additional attributes that are not necessary for unique identification
Foreign key
An attribute, or set of attributes, within one relation that matches the candidate key of some (possibly the same) relation.
Referential integrity
If a foreign key exists in a relation, either the foreign key value must match a candidate key value of some tuple in its home relation or the foreign key value must be wholly null.
entity integrity
In a base relation, no attribute of a primary key can be null. If we allow a null for any part of a primary key, we are implying that not all the attributes are needed to distinguish between tuples, which contradicts the definition of the primary key.
Null
Represents a value for an attribute that is currently unknown or is not applicable for this tuple. The logical value "unknown." Represents the absence of a value.
Primary key
The candidate key that is selected to identify tuples uniquely within the relation.
View
The dynamic result of one or more relational operations operating on the base relations to produce another relation. A ______ is a virtual relation that does not necessarily exist in the database but can be produced upon request by a particular user, at the time of request. is a relation that appears to the user to exist, can be manipulated as if it were a base relation, but does not necessarily exist in storage in the sense that the base relations do (although its definition is stored in the system catalog)
intension
The structure of a relation, together with a specification of the domains and any other restrictions on possible values, is sometimes called its ________, which is usually fixed, unless the meaning of a relation is changed to include additional attributes.
extension (or state)
The tuples are called the _________ of a relation, which changes over time.
A candidate key has two properties
Uniqueness. Irreducibility.
Updating Views Restrictions
Updates are allowed through a view defined using a simple query involving a single base relation and containing either the primary key or a candidate key of the base relation.
Updating Views Restrictions
Updates are not allowed through views involving aggregation or grouping operations.
Updating Views Restrictions
Updates are not allowed through views involving multiple base relations.
simplify
View mechanism can ______ complex operations on the base relations.
customized
View mechanism permits users to access data in a way that is ________ to their needs, so that the same data can be seen by different users in different ways, at the same time.
dynamic
Views are _______, meaning that changes made to the base relations that affect the view are immediately reflected in the view.
composite key
When a key consists of more than one attribute, we call it a ________
underlying base relation
if a view is updated, then the ________ should reflect the change.
An attribute
is a named column of a relation. May be referred to as fields.
A tuple
is a row of a relation. May be referred to as records.
A domain
is the set of allowable values for one or more attributes.
The degree of a relation
the number of attributes it contains.
The cardinality of a relation
the number of tuples it contains
Classes of views
theoretically not updatable, theoretically updatable, and partially updatable
All updates
to a base relation should be immediately reflected in all views that reference that base relation.
logical data independence
views allow a more significant type of _________ that supports the reorganization of the conceptual schema.
Base relation
A named relation corresponding to an entity in the conceptual schema, whose tuples are physically stored in the database.
Relation schema
A named relation defined by a set of attribute and domain name pairs.
unary relation or one-tuple
A relation with only one attribute would have degree one and be called a __________.
ternary
A relation with three attributes is called ________.
binary
A relation with two attributes is called _________.
Relational database schema
A set of relation schemas, each with a distinct name.
alternate keys
The candidate keys that are not selected to be the primary key are called ________.
integrity rules
two important ___________ for the relational model are known as entity integrity and referential integrity.
Candidate key
A superkey such that no proper subset is a superkey within the relation.
General constraints
Additional rules specified by the users or database administrators of a database that define or constrain some aspect of the enterprise.
security
View mechanism provides a powerful and flexible ________ mechanism by hiding parts of the database from certain users. Users are not aware of the existence of any attributes or tuples that are missing from the view.
A relation
a table with columns and rows. ______ is represented as a two-dimensional table in which the rows of the table correspond to individual records and the table columns correspond to attributes. May be referred to as a file
virtual or derived relation
a view is a ___________: a relation that does not necessarily exist in its own right, but may be dynamically derived from one or more base relations.
domain constraints
because every attribute has an associated domain, there are constraints (called _________) that form restrictions on the set of values allowed for the attributes of relations.
an external model
can consist of both base (conceptual-level) relations and views derived from the base relations.
Cartesian product
of two sets, written D 1 X D 2 , is the set of all ordered pairs such that the first element is a member of D 1 and the second element is a member of D 2 .
relational keys
one or more attributes that uniquely identifies each tuple in a relation.
