The Relational Model

Tables, Rows, and Columns

The relational model organizes data into tables (also called relations). Each table represents an entity — students, courses, orders, products. A table has columns (attributes) that define what data is stored, and rows (tuples) that represent individual records.

Key Terminology

  Table = Relation (e.g., Students)
  Row   = Tuple   (e.g., one student's data)
  Column = Attribute (e.g., Name, Age, Email)

  Students Table
  ┌────┬─────────┬─────┬──────────────────────┐
  │ ID │ Name    │ Age │ Email                │
  ├────┼─────────┼─────┼──────────────────────┤
  │ 1  │ Alice   │ 25  │ alice@example.com    │
  │ 2  │ Bob     │ 22  │ bob@example.com      │
  │ 3  │ Charlie │ 23  │ charlie@example.com  │
  └────┴─────────┴─────┴──────────────────────┘
    ↑     ↑       ↑           ↑
  Primary Key  Column definitions

Keys

Keys are fundamental to the relational model. They uniquely identify rows and establish relationships between tables.

• Primary Key — A column (or set of columns) that uniquely identifies each row. No two rows can have the same primary key value. Cannot be NULL.
• Foreign Key — A column in one table that references the primary key of another table. This is how relationships between tables are established.
• Composite Key — A primary key made up of two or more columns together.
• Candidate Key — Any column that could potentially be a primary key (unique and non-null).

  Students                    Enrollments
  ┌────┬─────────┐           ┌────┬─────────┬──────┐
  │ ID │ Name    │           │SID │CourseID │Grade │
  ├────┼─────────┤           ├────┼─────────┼──────┤
  │ 1  │ Alice   │◄──────────│ 1  │ 101     │ A    │
  │ 2  │ Bob     │◄──┐       │ 2  │ 101     │ B    │
  │ 3  │ Charlie │   │       │ 1  │ 103     │ A    │
  └────┴─────────┘   │       │ 3  │ 103     │ B+   │
                     └───────│ 3  │ 102     │ A-   │
                             └────┴─────────┴──────┘
  ID = Primary Key         SID = Foreign Key → Students.ID
                           CourseID = Foreign Key → Courses.ID

Relationships

• One-to-One (1:1) — Each row in Table A relates to exactly one row in Table B. (e.g., Person → Passport)
• One-to-Many (1:N) — Each row in Table A relates to many rows in Table B. (e.g., Department → Employees)
• Many-to-Many (M:N) — Rows in Table A relate to many rows in Table B, and vice versa. Implemented using a junction table. (e.g., Students ↔ Courses)