Database keys are the backbone of relational databases. They provide data integrity and seamless relationships between tables. Database keys are crucial for fetching records on any website. On an e-commerce website, where you manage customers’ data or track orders for multiple records, choosing the right keys can significantly impact database performance and consistency.
In this blog, we’ll explore Primary Key, Foreign Key, Unique Key, Composite Key, Candidate Key, Alternate Key, Surrogate Key, Super Key, Composite Foreign Key, and Default Key with real-world e-commerce examples.
1. Primary Key
Definition:
A Primary Key uniquely identifies each record in a table. It cannot contain NULL values and ensures every record is distinct.
Example: Customer Table
CREATE TABLE Customer (
CustomerID INT PRIMARY KEY,
FirstName NVARCHAR(100),
LastName NVARCHAR(100),
Email NVARCHAR(100) UNIQUE
);Use Case:
In an e-commerce platform, CustomerID ensures each customer record is unique and prevents duplication.
2. Foreign Key
Definition:
A Foreign Key creates a relationship between two tables by referencing the Primary Key in another table.
Example: Customer and Order Tables
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
CustomerID INT,
OrderDate DATETIME,
FOREIGN KEY (CustomerID) REFERENCES Customer(CustomerID)
);Use Case:
This relationship links orders to the customers who placed them. It will help maintain data integrity across the system.
3. Unique Key
Definition:
A Unique Key ensures that all values in a column are distinct but allows a single NULL value.
Example: Ensuring Unique Emails
ALTER TABLE Customer ADD CONSTRAINT UQ_Email UNIQUE (Email);Use Case:
Unique keys can be used to ensure email addresses in the Customer table are not duplicated.
4. Composite Key
Definition:
A Composite Key combines two or more columns to create a unique identifier for a record.
Example: Order Details Table
CREATE TABLE OrderDetails (
OrderID INT,
ProductID INT,
Quantity INT,
PRIMARY KEY (OrderID, ProductID),
FOREIGN KEY (OrderID) REFERENCES Orders(OrderID)
);Use Case:
In an e-commerce system, OrderID and ProductID together uniquely identify each product in an order.
5. Candidate Key
Definition:
A Candidate Key is any column or combination of columns that can uniquely identify a record.
Example: Candidate Keys in the Customer Table
Columns like CustomerID, Email, or PhoneNumber could all serve as Candidate Keys.
Use Case:
In practice, the best Candidate Key for the Customer table is selected as the Primary Key, such as CustomerID.
6. Alternate Key
Definition:
An Alternate Key is any Candidate Key not chosen as the Primary Key.
Example:
If CustomerID is the Primary Key, Email becomes an Alternate Key.
ALTER TABLE Customer ADD CONSTRAINT AK_Email UNIQUE (Email);Use Case:
Alternate Keys provide additional constraints to enforce data integrity, such as ensuring unique email addresses.
7. Surrogate Key
Definition:
A Surrogate Key is an artificially generated unique identifier, often used when no natural key exists.
Example: Efficient Indexing
CREATE TABLE Product (
ProductID INT IDENTITY(1,1) PRIMARY KEY,
ProductName NVARCHAR(100),
Price DECIMAL(10, 2)
);Use Case:
In large datasets, ProductID as a surrogate key simplifies indexing and querying, avoiding reliance on complex natural keys.
8. Super Key
Definition:
A Super Key is any set of columns that can uniquely identify a record, including Candidate Keys and Composite Keys.
Example: Super Key in the Customer Table
CustomerID is a Super Key, and so is the combination of Email and PhoneNumber.
Use Case:
Super Keys help determine all possible unique identifiers in a table.
9. Composite Foreign Key
Definition:
A Composite Foreign Key references multiple columns in a Primary Key of another table.
Example: Composite Key in Order Details
CREATE TABLE OrderDetails (
OrderID INT,
ProductID INT,
FOREIGN KEY (OrderID, ProductID) REFERENCES Orders(OrderID, ProductID)
);Use Case:
Composite Foreign Keys maintain relationships between complex multi-column Primary Keys.
10. Default Key
Definition:
A Default Key is a column with a default value that helps maintain data consistency.
Example: Default Order Status
CREATE TABLE Orders (
OrderID INT PRIMARY KEY,
CustomerID INT,
OrderStatus NVARCHAR(50) DEFAULT 'Pending'
);Use Case:
Default values like Pending for OrderStatus reduce the risk of missing or inconsistent data.
Best Practices for Choosing Keys
- Understand Requirements: Identify the key that best ensures data uniqueness and supports querying needs.
- Avoid Over-Complexity: Use Surrogate Keys for simplicity in large datasets.
- Use Defaults Wisely: Apply Default Keys for consistency in frequently used fields.
- Optimize Relationships: Use Foreign and Composite Keys to maintain relational integrity.
Conclusion: Keys to a Robust Database
SQL Server keys are essential for maintaining the integrity and efficiency of any relational database. By understanding the types of keys and their applications, you can design robust e-commerce systems capable of handling complex data relationships.
Apply these principles to your projects to unlock SQL Server’s true potential and build systems that scale seamlessly!