Relational Schema

Relational Schema

A named relation defined by a set of attributes.

NAME OF RELATION(primary_key, foreign_key, attribute)

Format for relational schema

Relational Database

• Collection of multiple normalized relations

• Physical implementation of the relational data model andnormally one uses the services of the DBMS

Metadata (“data about data”)

referred to as the data dictionary or the system catalog or simply catalog.

Foreign Key (FK)

• Is a column or set of columns whose value matches the primary key of another table

• You can think of a foreign key as a copy of primary key from another relational table.

Constraints

• Restrictions on the set of values allowed for attributes of relations

• Allows to enforce rules in your database. These rules may affect business logic, database integrity, and/or table structures

Integrity Rules

Constraints or restrictions that apply to all instances of the database

Entity Integrity Rule (PK Constraint)

• States that the value of the primary key can never be a null value

• Requires that insert, update, and delete operations maintain the uniqueness and existence of all primary keys.

Null Value

• Represents a value for an attribute that is currently unknown for a tuple

• Not the same as zero or text string filled with spaces

• Absence of a value

Referential Integrity Rule (FK Constraint)

States that if a relational table has a foreign key, then every value of the foreign key must be null or match the values in the relational table in which that foreign key is a primary key

NOT NULL Constraint

Allow you to specify that a column may not contain NULL values

DEFAULT Constraint

• Allow you to specify a value that the database will use to populate fields that are left blank in the input source

• Replacement for the use of NULL values

• Great way to predefine common data elements

UNIQUE Constraint

Constraints you to specify that a column may not contain duplicate values

CHECK Constraint

• Allow you to limit the types of data that users may insert in a database

• Go beyond data types and allow you to define the specific values that may be included in a column.