Page 4 - Relational Databases

 If Branch relation have 5 number of tuples, the cardinality of Branch relation is 5.
 Change as the number of tuples is added or deleted.

Relational Schema
A named relation/ table schema is a set of related attributes defining/describing the table.
Characteristics:
 Each attribute has a distinct name
 Order of attributes has no significance
 Order of tuples has no significance, theoretically

Relational Database schema
A sets of relation schema for an organization.
The relation schema normally represented as follows:
(The primary key is underlined.)

Relation Keys
 Refers to the important attribute in an entity.
 Determine the uniqueness of a row in given table.
 Identifiers for each row.
 An attribute or more than one attributes can be declared as keys depending on
situations.
 Types of keys:
 Primary Key
 Candidate Key
 Alternate Key
 Foreign key

Primary Key

 Is an attributes that uniquely identify each rows.
 Each table must have primary key.
 Cannot be NULL value to maintain Entity Integrity.
 Primary key is the one that officially declared as the row identifier inside a specific
table.
 Primary key for a table:
Student(studentID, firstname, lastname, coursed): studentID

Candidate key
 A candidate key is a single field or the least combination of fields that uniquely
identifies each record in the table.
 There can be more than one candidate keys in a relation.