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What is SQL Constraints?

Anubhav Sharma 249 21 Apr 2026

1 Answers

Anubhav Sharma 21 Apr 2026

SQL Constraints are rules applied to table columns to control what data can be stored in a database. They help maintain data accuracy, consistency, and integrity.

What are SQL Constraints?

Constraints act like validation rules at the database level.
They ensure that only valid data is inserted, updated, or deleted.

Why Constraints are Important

Prevent invalid data entry
Maintain data integrity
Enforce business rules
Improve data consistency across tables

Types of SQL Constraints

1. NOT NULL

Ensures that a column cannot have NULL (empty) values

CREATE TABLE Users (
    Id INT,
    Name VARCHAR(100) NOT NULL
);

2. UNIQUE

Ensures all values in a column are different

CREATE TABLE Users (
    Email VARCHAR(100) UNIQUE
);

3. PRIMARY KEY

Uniquely identifies each row
Cannot be NULL
Combines NOT NULL + UNIQUE

CREATE TABLE Users (
    Id INT PRIMARY KEY
);

4. FOREIGN KEY

Creates a relationship between two tables

CREATE TABLE Orders (
    OrderId INT PRIMARY KEY,
    UserId INT,
    FOREIGN KEY (UserId) REFERENCES Users(Id)
);

5. CHECK

Ensures values meet a specific condition

CREATE TABLE Employees (
    Age INT CHECK (Age >= 18)
);

6. DEFAULT

Sets a default value if none is provided

CREATE TABLE Users (
    Status VARCHAR(20) DEFAULT 'Active'
);

Example with Multiple Constraints

CREATE TABLE Students (
    Id INT PRIMARY KEY,
    Name VARCHAR(100) NOT NULL,
    Email VARCHAR(100) UNIQUE,
    Age INT CHECK (Age >= 18),
    Status VARCHAR(20) DEFAULT 'Active'
);

Table-Level vs Column-Level Constraints

Column-Level → Applied directly to a column
Table-Level → Applied to the whole table (useful for composite keys)

CREATE TABLE Enrollment (
    StudentId INT,
    CourseId INT,
    PRIMARY KEY (StudentId, CourseId)
);

Advantages

Ensures data correctness
Reduces bugs in applications
Improves database reliability

In One Line

SQL Constraints are rules that enforce valid and consistent data in a database.

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