<<–2/”>a href=”https://exam.pscnotes.com/5653-2/”>p>primary and foreign keys, including a table of differences, advantages, disadvantages, similarities, and FAQs:
Introduction
In the world of relational databases, keys are crucial Elements for maintaining data Integrity and establishing relationships between tables. Two fundamental types of keys are the primary key and the foreign key. Let’s delve deeper into understanding their roles and how they differ.
Primary Key: A primary key is a column (or combination of columns) that uniquely identifies each row in a table. It acts like a fingerprint, ensuring that no two records have the same value in the primary key column(s).
Foreign Key: A foreign key is a column (or combination of columns) in one table that refers to the primary key of another table. It establishes a link between the two tables, enabling you to create relationships and enforce referential integrity.
Key Differences: Primary Key vs. Foreign Key
| Feature | Primary Key | Foreign Key |
|---|---|---|
| Purpose | Uniquely identifies each row in a table | Establishes a relationship between tables |
| Uniqueness | Must be unique (cannot have duplicate values) | Can have duplicate values (not enforced) |
| Null Values | Cannot be NULL | Can be NULL (if allowed in the column definition) |
| Number Allowed Per Table | Only one per table | Multiple foreign keys allowed per table |
| Referential Integrity | Enforces within the same table | Enforces between tables (child to parent) |
Advantages and Disadvantages
Primary Key
- Advantages:
- Ensures data integrity and uniqueness.
- Provides a quick and efficient way to access and retrieve specific records.
- Forms the basis for relationships with other tables.
- Disadvantages:
- Can be complex to choose the right column(s) for the primary key.
- Changing the primary key can be challenging once data is populated.
Foreign Key
- Advantages:
- Maintains referential integrity by ensuring data consistency between related tables.
- Prevents the creation of orphan records (records in the child table that don’t have a corresponding parent record).
- Facilitates data navigation and querying across multiple tables.
- Disadvantages:
- Can impact performance if not properly indexed.
- Requires careful design to avoid circular references or cascading updates/deletes.
Similarities
- Both primary and foreign keys are used to define relationships between data.
- Both help maintain data integrity by enforcing constraints on the values allowed in a column.
- Both can be composed of a single column or multiple columns.
Frequently Asked Questions (FAQs)
1. Can a foreign key refer to the primary key of the same table?
Yes, this is possible and is known as a self-referencing foreign key. It’s often used to represent hierarchical relationships (e.g., an employee table where one employee is the manager of other employees).
2. Can a table have more than one primary key?
No, a table can only have one primary key. However, the primary key can be composed of multiple columns (known as a composite primary key).
3. What happens if I try to delete a record from a table that has a foreign key relationship?
The Database will enforce referential integrity. By default, you cannot delete a record from the parent table if there are corresponding records in the child table. You can change this behavior using different “on delete” actions (e.g., cascade, set null).
4. How do I choose the right columns for a primary key?
Look for columns (or combinations of columns) that:
* Uniquely identify each row.
* Are unlikely to change over time.
* Are as small as possible to improve performance.
5. Can a foreign key contain NULL values?
Yes, a foreign key column can contain NULL values if the column is defined as nullable. This means that the record in the child table doesn’t have a corresponding parent record.
Let me know if you’d like more details on any of these topics!