SQL PRIMARY KEY

The PRIMARY KEY constraint uniquely identifies each record in a table. It ensures that no duplicate or NULL values exist in the column(s) defined as the primary key.

 

Key Features of SQL PRIMARY KEY

• Uniquely identifies each row in a table.
• Does not allow NULL values.
• Automatically creates an index for faster lookups.
• A table can have only one primary key.
• The primary key can be a single column or a combination of multiple columns (composite key).

 

SQL PRIMARY KEY Syntax

Single Column Primary Key

CREATE TABLE Employees (
    EmployeeID INT PRIMARY KEY,
    Name VARCHAR(100),
    Department VARCHAR(50)
);

• The EmployeeID column is the primary key.

Composite Primary Key (Multiple Columns)

CREATE TABLE Orders (
    OrderID INT,
    ProductID INT,
    Quantity INT,
    PRIMARY KEY (OrderID, ProductID)
);

• The combination of OrderID and ProductID ensures uniqueness.

 

Adding a PRIMARY KEY to an Existing Table

Adding a Primary Key After Table Creation

ALTER TABLE Employees  
ADD PRIMARY KEY (EmployeeID);

 

Removing a PRIMARY KEY

ALTER TABLE Employees  
DROP PRIMARY KEY;

 

Summary

• A PRIMARY KEY uniquely identifies each record in a table.
• It does not allow NULL or duplicate values.
• Can be defined on one or multiple columns.
• Can be added or removed using ALTER TABLE.