SQL PRIMARY KEY Constraint

The PRIMARY KEY constraint in SQL uniquely identifies each record in a table and ensures strong data integrity. It prevents duplicate and NULL values, making it essential for reliable relational database design.

• Ensures all values are unique.
• Does not allow NULL values.
• Only one primary key per table (can be composite).
• Automatically creates a unique index for faster searches.

Query:

CREATE TABLE employees (
    emp_id INT PRIMARY KEY,
    emp_name VARCHAR(50),
    department VARCHAR(50)
);

-- Insert data into employees table
INSERT INTO employees (emp_id, emp_name, department)
VALUES
    (101, 'Bob', 'Sales'),
    (102, 'Lucas', 'HR');

Output:

• EmpID is the primary key, so it must be unique and cannot be NULL.
• If you try inserting duplicate or NULL EmpID values, the database will throw an error.

Query:

-- Duplicate primary key value (EmpID = 101 already exists)
INSERT INTO Employees VALUES (101, 'Alice', 'IT');

-- NULL not allowed in primary key column
INSERT INTO Employees VALUES (NULL, 'Emma', 'Finance');

Error: