Making Sense of Your Files: Understanding Database Keys
- These keys use existing data in the table itself, like a customer ID number, social security number (though for privacy reasons this wouldn't be ideal!), or a product code.
- Pros: Easy for humans to understand what the key refers to, and you might already have these unique identifiers.
- Cons:
- Can be problematic if the natural identifier isn't truly unique (e.g., two customers with the same name).
- If the identifier changes (e.g., a customer gets a new ID number), it can break connections (like foreign keys) in other tables.
Surrogate Keys:
- These are database-generated numbers with no inherent meaning. They are simply unique identifiers for each record. Often they are auto-incrementing integers (1, 2, 3, etc.).
- Pros:
- Guaranteed to be unique, avoiding conflicts.
- Remain stable even if the underlying data changes.
- Smaller size compared to some natural keys which can improve performance.
- Cons: Don't have any inherent meaning for humans, making it harder to understand what a specific record represents.
CREATE TABLE Customers (
customer_id VARCHAR(255) PRIMARY KEY, -- Maybe a customer number
first_name VARCHAR(255),
last_name VARCHAR(255)
);
This example uses customer_id
(assuming it's unique) as a natural key. Potential issues arise if this ID isn't truly unique (e.g., duplicate entries) or if the ID changes for a customer, causing issues with foreign keys in other tables.
Surrogate Key (Reliable but Less Meaningful):
CREATE TABLE Customers (
customer_id INT AUTO_INCREMENT PRIMARY KEY,
first_name VARCHAR(255),
last_name VARCHAR(255)
);
This example uses customer_id
as a surrogate key. It's an auto-incrementing integer, guaranteed to be unique. However, it lacks inherent meaning for humans.
Combination Approach (Both Keys):
CREATE TABLE Customers (
customer_id INT AUTO_INCREMENT PRIMARY KEY,
natural_id VARCHAR(255) UNIQUE, -- Maybe a customer number (enforced unique)
first_name VARCHAR(255),
last_name VARCHAR(255)
);
This approach combines both. customer_id
is the reliable surrogate key, while natural_id
(enforced unique) allows referencing by a more human-readable identifier.
Clustered Key:
- This isn't strictly an alternate key definition, but a way to store data physically based on the primary key order.
- If your queries frequently retrieve data sorted by the primary key, a clustered key can significantly improve performance by storing related records together on disk.
- The chosen key for clustering should ideally be used in queries often.
Hashed Key:
- This involves using a mathematical function (hash function) on the chosen key value to generate a unique identifier.
- This approach is particularly useful for very large datasets where fast lookups are essential.
- However, hashed keys aren't ordered and cannot be used for efficient range-based queries.
GUID (Globally Unique Identifier):
- A GUID is a 128-bit value mathematically guaranteed to be unique across systems, even if generated independently.
- These are useful when there's a need for unique identifiers across multiple databases or systems.
- The downside is their larger size compared to integer-based keys, potentially impacting storage and performance.
Remember, the best approach depends on your specific data and query patterns. Consider factors like:
- Uniqueness: How critical is it to guarantee unique record identification?
- Performance: How often are lookups performed on the primary key, and in what order?
- Human Readability: Is it important for users to understand the meaning behind the key value?
- Storage Efficiency: How much storage space can you allocate for the key field?
database database-design primary-key