BTE 423 Quiz 2
Views
A virtual relation that does not necessarily actually exist in the database but is produced upon request, at time of request. Contents of a view are defined as a query on one or more base relations. Views are dynamic, meaning that changes made to base relations that affect view attributes are immediately reflected in the view.
General Constraints
Additional rules specified by users or database administrators that define or constrain some aspect of the enterprise.
Updating Views
All updates to a base relation should be immediately reflected in all views that reference that base relation. If view is updated, underlying base relation should reflect change. There are restrictions on types of modifications that can be made through views: Classes of views are defined as:
Superkey
An attribute, or set of attributes, that uniquely identifies a tuple within a relation.
Foreign Key
Attribute, or set of attributes, within one relation that matches candidate key of some (possibly same) relation.
Views (Type?)
Base Relation View
Primary Key
Candidate key selected to identify tuples uniquely within relation.
Alternate Keys
Candidate keys that are not selected to be primary key.
Consider three sets D1, D2, D3 with Cartesian Product D1 ´ D2 ´ D3; e.g.
D1 = {1, 3} D2 = {2, 4} D3 = {5, 6} D1 ´ D2 ´ D3 = {(1,2,5), (1,2,6), (1,4,5), (1,4,6), (3,2,5), (3,2,6), (3,4,5), (3,4,6)} Any subset of these ordered triples is a relation.
Cartesian product of n sets (D1, D2, . . ., Dn) is:
D1 ´ D2 ´ . . . ´ Dn = {(d1, d2, . . . , dn) | d1 ÎD1, d2 ÎD2, . . . , dnÎDn} usually written as: n XDi i = 1
View
Dynamic result of one or more relational operations operating on base relations to produce another relation.
Referential Integrity
If foreign key exists in a relation, either foreign key value must match a candidate key value of some tuple in its home relation or foreign key value must be wholly null.
Entity Integrity
In a base relation, no attribute of a primary key can be null.
Base Relation
Named relation corresponding to an entity in conceptual schema, whose tuples are physically stored in database.
Relation schema
Named relation defined by a set of attribute and domain name pairs.
Integrity Constraints
Null Entity Integrity Referential Integrity General Constraints
A relation is a table with columns and rows.
Only applies to logical structure of the database, not the physical structure.
Purpose of Views
Provides powerful and flexible security mechanism by hiding parts of database from certain users. Permits users to access data in a customized way, so that same data can be seen by different users in different ways, at same time. Can simplify complex operations on base relations.
Properties of Relations
Relation name is distinct from all other relation names in relational schema. Each cell of relation contains exactly one atomic (single) value. Each attribute has a distinct name. Values of an attribute are all from the same domain.
Null
Represents value for an attribute that is currently unknown or not applicable for tuple. Deals with incomplete or exceptional data. Represents the absence of a value and is not the same as zero or spaces, which are values.
Relational database schema
Set of relation schemas, each with a distinct name.
Candidate Key
Superkey (K) such that no proper subset is a superkey within the relation. In each tuple of R, values of K uniquely identify that tuple (uniqueness). No proper subset of K has the uniqueness property (irreducibility).
There are restrictions on types of modifications that can be made through views:
Updates are allowed if query involves a single base relation and contains a candidate key of base relation. Updates are not allowed involving multiple base relations. Updates are not allowed involving aggregation or grouping operations.
Relational Database is
a collection of normalized relations with distinct relation names.
Attribute is
a named column of a relation.
Any set of n-tuples from this Cartesian product is
a relation on the n sets.
Tuple is
a row of a relation.
A relation is
a table with columns and rows.
Order of tuples has
no significance, theoretically.
Order of attributes has
no significance.
Any subset of Cartesian product is a
relation; e.g. R = {(2, 1), (4, 1)}
May specify which pairs are in relation using some condition for selection; e.g.
second element is 1: R = {(x, y) | x ÎD1, y ÎD2, and y = 1} first element is always twice the second: S = {(x, y) | x ÎD1, y ÎD2, and x = 2y}
Consider two sets, D1 & D2, where D1 = {2, 4} and D2 = {1, 3, 5}. Cartesian product, D1´D2, is
set of all ordered pairs, where first element is member of D1 and second element is member of D2. D1 ´ D2 = {(2, 1), (2, 3), (2, 5), (4, 1), (4, 3), (4, 5)}
Degree is
the number of attributes in a relation.
Cardinality is
the number of tuples in a relation.
Domain is
the set of allowable values for one or more attributes.
Classes of views are defined as:
theoretically not updateable; theoretically updateable; partially updateable.
Each tuple is distinct;
there are no duplicate tuples.
Alternative way is
to find all combinations of elements with first from D1 and second from D2.
