The Role of Tables in Relational Databases
Relational databases organize data into tables (also known as entities), where each table represents a specific category of information.
- Tables consist of rows (records) and columns (attributes).
- Each row represents a unique instance of the entity, while each column stores a specific attribute of that entity.
Understanding the structure of tables is crucial for designing efficient databases that maintain data integrity and support complex queries.
Keys: The Backbone of Relational Databases
Keys are essential for defining relationships between tables and ensuring data integrity.
Primary Keys
- A primary key is a unique identifier for each record in a table.
- It ensures that no two rows have the same value in the primary key column.
- Example: In a Student table, StudentID could be the primary key.
- Choose primary keys that are stable and unlikely to change, such as unique IDs, rather than attributes like names or email addresses.
- Most RDBMS will have a mechanism to generate unique keys or simply generate auto incremented integers which work well for PK IDs.
Foreign Keys
- A foreign key is a column (or set of columns) in one table that references the primary key of another table.
- It establishes a relationship between the two tables.
- Example: In an Enrollment table, StudentID might be a foreign key referencing the StudentID in the Student table.
- Think of a foreign key as a hyperlink in a document.
- Just as a hyperlink connects you to another webpage, a foreign key connects a record in one table to a related record in another table.
Composite Keys
- A composite key is a primary key composed of two or more columns.
- It is used when a single column is not sufficient to uniquely identify a record.
- Example: In a CourseEnrollment table, a composite key might consist of StudentID and CourseID.
Concatenated Keys
- Concatenated keys are similar to composite keys but are specifically used to create unique identifiers by combining multiple columns.
- They are often used in junction tables to represent many-to-many relationships.
