Relational Database Model Concepts
Informal names used for relations
tables, data files
Properties of Relations
1. There are no duplicate tuples in a relation 2. Tuples are un-ordered 3. All attribute values are atomic
Attribute
A characteristic of data. A real world data feature, modeled in the database, will be represented by an attribute. An attribute has to have a name, so you can refer to that feature, and the name has to be as relevant as possible for that feature. For example a person can have the attributes of Name, Sex, Date of Birth. Informal terms used to define an attribute as column in a table and field in a data file.
Candidate Keys
A unique identifier for the tuples of a relations. By definition, every relation has at least 1 candidate key ( the first property of a relation). In practice, most relations have multiple keys.
Tuple
An ordered set of values that describe data characteristics at one moment in time. Other names: row in a table and record in a data file.
database schema
a formal description of all the database relations and all the relationships existing between them.
What do Keys do?
define identifiers for a relation's tuples. The keys are used to enforce rules and/or constraints on the database data. Those constraints are essential for maintaining data consistency and correctness.
Entity Integrity Constraint
no attribute participating in the primary key of a relation is allowed to accept null values.
semantic integrity constraint
refers to the correctness of the meaning of the data. For example, the street number attribute value from the OWNERS relation must be positive, because the real-world street numbers are positive. Semantic integrity constraints must be specified typically by a database administrator and must be maintained in the system catalog or dictionary. The DBMS's user interactions to ensure that the constraints are in fact respected. Relational DBMS permits several types of semantic integrity constraints such as domain constraint, null constraint, unique constraint, and check constraint
