Microsoft SQL Server Tutorial Series: Beginner to Expert

Part 6: SQL Constraints – PK, FK, UNIQUE, NOT NULL

Welcome back! In this part of our SQL Server series, we’ll explore SQL Constraints, the rules that help ensure your database is accurate, consistent, and reliable.

📚 What You’ll Learn

What SQL Constraints are and why they matter
How to use PRIMARY KEY, FOREIGN KEY, UNIQUE, and NOT NULL
How to add constraints while creating or altering tables
Real-world examples using Students and Courses tables

🔐 What Are SQL Constraints?

Constraints are rules applied to table columns to control the type of data that can be stored. They help enforce data integrity and prevent bad or duplicate data from being inserted.

🧱 1. PRIMARY KEY Constraint

The PRIMARY KEY uniquely identifies each row in a table. It must be:

Unique (no duplicates)
Not NULL

CREATE TABLE Students (
    StudentID INT PRIMARY KEY,
    FirstName VARCHAR(50),
    LastName VARCHAR(50)
);

💡 You can only have one primary key per table, but it can consist of one or more columns (composite key).

🔗 2. FOREIGN KEY Constraint

The FOREIGN KEY establishes a link between two tables. It ensures that the value in one table matches a value in another table.

CREATE TABLE Courses (
    CourseID INT PRIMARY KEY,
    CourseName VARCHAR(100)
);

CREATE TABLE Enrollments (
    EnrollmentID INT PRIMARY KEY,
    StudentID INT,
    CourseID INT,
    FOREIGN KEY (StudentID) REFERENCES Students(StudentID),
    FOREIGN KEY (CourseID) REFERENCES Courses(CourseID)
);

💡 If a referenced value doesn’t exist in the parent table, SQL Server will throw an error — this maintains referential integrity.

🔁 3. UNIQUE Constraint

The UNIQUE constraint ensures that all values in a column are different.

CREATE TABLE Teachers (
    TeacherID INT PRIMARY KEY,
    Email VARCHAR(100) UNIQUE,
    Name VARCHAR(50)
);

💡 Unlike PRIMARY KEY, a table can have multiple UNIQUE constraints, but each column must still allow only unique values.

🚫 4. NOT NULL Constraint

NOT NULL means a column must have a value — it can’t be left empty.

CREATE TABLE Departments (
    DeptID INT PRIMARY KEY,
    DeptName VARCHAR(50) NOT NULL
);

💡 Use this for fields like names, dates, or emails that should never be blank.

🧾 SQL Cheat Sheet

-- PRIMARY KEY
CREATE TABLE Example (
    ID INT PRIMARY KEY
);

-- FOREIGN KEY
FOREIGN KEY (ColumnName) REFERENCES OtherTable(OtherColumn)

-- UNIQUE
Email VARCHAR(100) UNIQUE

-- NOT NULL
Name VARCHAR(50) NOT NULL

🌍 Real-World Example

In a school system:

StudentID in Students is a PRIMARY KEY
CourseID in Enrollments is a FOREIGN KEY
Email in Teachers must be UNIQUE
CourseName in Courses should be NOT NULL

📋 Quick Comparison Table

Constraint	Purpose	Allows NULL?	Allows Duplicates?
PRIMARY KEY	Uniquely identifies each row	No	No
FOREIGN KEY	Creates link between tables	Yes	Yes
UNIQUE	Ensures all values are different	Yes	No
NOT NULL	Requires a value in the column	No	Yes

✅ Best Practices

Always use PRIMARY KEY for every table.
Use FOREIGN KEYS to define relationships.
Combine UNIQUE + NOT NULL to enforce data quality.
Name your constraints explicitly for clarity (e.g., CONSTRAINT FK_StudentID).

🧠 Summary

Today, you learned how constraints help maintain the accuracy and integrity of your data:

PRIMARY KEY – Uniquely identifies each record
FOREIGN KEY – Creates table relationships
UNIQUE – Ensures no duplicates
NOT NULL – Requires non-empty values

Constraints are essential in professional database design — now you know how to use them confidently! 🎉

🔗 Previous & Next Posts

Questions or feedback? Leave a comment below and share your progress! 🙌

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SQL Constraints – PRIMARY KEY, FOREIGN KEY, UNIQUE, NOT NULL Explained