Primary Key

Definition

A primary key is a unique identifier for each record in a database table, ensuring that no two rows can have the same key value. It plays a crucial role in maintaining the integrity of data by preventing duplicate entries and enabling efficient data retrieval.

5 Must Know Facts For Your Next Test

1. A primary key must contain unique values for each record, meaning it cannot have duplicates.
2. The primary key cannot contain NULL values, ensuring that every record has a defined identifier.
3. It often consists of a single column but can also be made up of multiple columns (composite key) to ensure uniqueness.
4. Using a primary key helps to establish relationships between different tables within the database, allowing for better organization and data integrity.
5. Choosing the right primary key is critical; it should be stable (not change over time) and simple to use for indexing and lookups.

Review Questions

• How does the choice of a primary key impact database normalization and data integrity?
  • Choosing an appropriate primary key is fundamental for achieving database normalization and ensuring data integrity. A well-chosen primary key uniquely identifies records, which helps in avoiding duplication and enforcing relationships across tables. This contributes to normal forms like 1NF, 2NF, and beyond by eliminating redundancy and ensuring that data dependencies are logically organized.
• Discuss how primary keys facilitate relationships between tables and how this relates to referential integrity.
  • Primary keys are essential for establishing relationships between tables through foreign keys. When a foreign key references a primary key in another table, it creates a link that maintains referential integrity. This means that any foreign key value must correspond to an existing primary key value, preventing orphaned records and ensuring consistent data across related tables.
• Evaluate the implications of choosing a poorly defined primary key on database performance and integrity.
  • Choosing a poorly defined primary key can severely impact database performance and integrity. If the primary key is not unique or stable, it can lead to duplicate records and inconsistencies in data retrieval. Furthermore, this can complicate relationships with other tables, making joins inefficient and causing additional overhead during insertions or updates. A weak primary key undermines the overall structure of the database, leading to issues like redundancy, reduced query performance, and potential data loss.

Related terms

Foreign Key: A foreign key is a field in one table that links to the primary key of another table, establishing a relationship between the two tables.

Candidate Key: A candidate key is a set of attributes that can uniquely identify a record in a table, from which one is chosen to be the primary key.

Unique Constraint:

A unique constraint ensures that all values in a column are different from each other, similar to a primary key, but it allows for NULL values.