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.

1. 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.

2. 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.

3. 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?