SQL ALTERNATE KEY

An Alternate Key is a candidate key that is not chosen as the Primary Key but can still uniquely identify a record in a table.

• When multiple attributes can uniquely identify records, only one is selected as the Primary Key.
• The remaining unique attributes act as Alternate Keys, providing alternative ways to identify records.

Query:

CREATE TABLE CustomerInfo (
    CustomerID INT PRIMARY KEY,         
    CustomerName VARCHAR(50),
    EmailAddress VARCHAR(100) UNIQUE,  
    PanNumber VARCHAR(20) UNIQUE,       
    City VARCHAR(50)
);


INSERT INTO CustomerInfo VALUES 
(1, 'James Carter', 'james.carter@example.com', 'PANA12345Z', 'New York'),
(2, 'Emily Watson', 'emily.watson@example.com', 'PANB67890Q', 'Los Angeles');

Output:

• EmailAddress and PanNumber act as Alternate Keys (unique but not the Primary Key).
• Both rows follow the uniqueness rules.

Syntax:

The ALTERNATE Keys in SQL are defined using the SQL constraint UNIQUE:

CREATE TABLE CustomerInfo (
    colum_1 datatype PRIMARY KEY,   -- Primary Key
    column_2 datatype ,
    column_3 datatype UNIQUE,    -- Alternate Key
    column_4 datatype UNIQUE,    -- Alternate Key
    . . . ,
);

Note: A candidate key should be a column that can uniquely identify any row in a table, and any of them are eligible to be selected as the Primary Key.

Creating an Alternate Key in SQL

An Alternate Key in SQL is created using the UNIQUE constraint to enforce uniqueness on a column.

Query:

CREATE TABLE Product (
    ProductID INT PRIMARY KEY,
    ProductName VARCHAR(50),
    ProductCode VARCHAR(20) UNIQUE      -- Alternate Key
);
CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    OrderNumber VARCHAR(30) UNIQUE,     -- Alternate Key
    ProductID INT,
    Quantity INT,
    FOREIGN KEY (ProductID) REFERENCES Product(ProductID)
);
-- Insert data into Product table
INSERT INTO Product VALUES
(101, 'Wireless Mouse', 'WMX-100'),
(102, 'Bluetooth Speaker', 'BTS-250');

-- Insert data into Order table
INSERT INTO Orders VALUES
(1, 'ORD-9001', 101, 2),
(2, 'ORD-9002', 102, 1);

Output:

• ProductID is the Primary Key in Product, while ProductCode is UNIQUE, making it the Alternate Key.
• OrderID is the Primary Key in Orders, and ProductID functions as a Foreign Key referencing Product.
• OrderNumber is UNIQUE, so it serves as the Alternate Key in the Orders table.

Primary Key Vs Alternate Key

Here are the detailed comparison between Primary and Alternate key: