Primary-Keys
In database design, a Primary-Key is a field or combination of fields that uniquely identifies each record in a database table. Here are key points about Primary-Keys:
- Uniqueness: The primary purpose of a Primary-Key is to ensure that no two rows in a table have the same key value, thus guaranteeing data integrity. Each value in the Primary-Key must be unique within the table.
- Non-Nullability: A Primary-Key cannot contain
NULL values. Each column in the Primary-Key must have a value.
- Minimal: A Primary-Key should be minimal, meaning it should contain only enough columns to uniquely identify each record. Overuse of columns can lead to inefficiencies.
- Clustering: In many database systems, the Primary-Key is used to physically organize the data within the table, often in a clustered index. This organization can improve data retrieval performance.
- Types:
- Single-Column Primary-Key: A single field serves as the Primary-Key. Common examples include an ID field like
customer_id or product_id.
- Composite Primary-Key: Multiple fields together form the Primary-Key. This is useful when a single field isn't enough to ensure uniqueness, for example, in a table linking students to courses, both
student_id and course_id might be needed.
- History and Evolution:
- Primary-Keys have been fundamental to relational database theory since the inception of the relational model by Edgar F. Codd in 1970. His work laid the foundation for SQL and modern database systems.
- Over time, as databases grew larger and more complex, the strategies for defining Primary-Keys evolved to handle scalability and performance issues.
- Implementation:
- Database Management Systems (DBMS) like MySQL, PostgreSQL, and Oracle Database have built-in mechanisms to enforce Primary-Key constraints.
- In SQL, a Primary-Key is typically defined with the
PRIMARY KEY constraint during table creation or alteration.
Here are some external resources for further reading:
Related topics: