In the context of the Relational Data Model, relations are tables that store data in a structured and organized manner. Relations represent the entities or concepts of the real-world domain being modeled. Here are the key concepts related to relations:

1. Relation Schema:

The relation schema defines the structure of a relation (table) and specifies the names of the attributes (columns) and their corresponding data types. It provides a blueprint for creating instances of the relation.

2. Relation Instance:

A relation instance, also known as a relation state or table instance, represents the actual data stored in a relation at a specific point in time. It consists of rows (tuples) and columns (attributes) that hold the actual data values.

3. Tuples (Rows):

Tuples, also referred to as rows, represent individual records or instances within a relation. Each tuple contains data values that correspond to the attributes defined by the relation schema. Each tuple is uniquely identified by a combination of attribute values, including the primary key.

4. Attributes (Columns):

Attributes, also known as columns, represent the properties or characteristics of an entity within a relation. Each attribute has a name and a data type that defines the kind of data it can hold, such as text, numbers, or dates. Each attribute in a relation must have a unique name.

5. Cardinality:

Cardinality refers to the number of tuples (rows) contained in a relation. It represents the number of instances of a particular entity represented by the relation. Cardinality can be one-to-one, one-to-many, many-to-one, or many-to-many, depending on the relationship between entities.

6. Degree:

Degree refers to the number of attributes (columns) present in a relation. It represents the number of properties or characteristics associated with the entity represented by the relation. A relation can be unary (degree 1), binary (degree 2), ternary (degree 3), or higher degree, depending on the number of attributes.

7. Referential Integrity:

Referential integrity is a constraint that ensures the consistency of relationships between relations. It requires that a foreign key value in one relation must match an existing primary key value in another relation. Referential integrity ensures that related data remains valid and consistent.

8. KEYS:

In Relational Database Management Systems (RDBMS), keys play a crucial role in defining and ensuring the integrity of data. Keys are used to identify and establish relationships between tables. The types of keys in RDBMS described in next page.

These concepts collectively define the structure, organization, and integrity of data within relations in the relational data model. They provide a foundation for designing and managing databases using the principles of the Relational Database Management System (RDBMS).

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