what is database constraints?

Database constraints are rules applied to database tables to make sure the data stored is accurate, consistent, and reliable.

Think of them as guardrails for your data — they prevent invalid or unwanted data from getting into the database.

Why database constraints are needed

They help to:

• Prevent duplicate or invalid data
• Maintain data integrity
• Enforce relationships between tables
• Reduce bugs caused by bad data

Common types of database constraints

1. PRIMARY KEY

• Uniquely identifies each row in a table
• Cannot be NULL
• No duplicate values

Example:

UserId INT PRIMARY KEY

2. FOREIGN KEY

• Creates a relationship between two tables
• Ensures values exist in the referenced table

Example:

UserId INT,
FOREIGN KEY (UserId) REFERENCES Users(UserId)

3. UNIQUE

• Ensures all values in a column are unique
• Allows only one NULL (depends on DB)

Example:

Email VARCHAR(100) UNIQUE

4. NOT NULL

• Prevents NULL values in a column

Example:

Name VARCHAR(50) NOT NULL

5. CHECK

• Validates data using a condition

Example:

Age INT CHECK (Age >= 18)

6. DEFAULT

• Assigns a default value if none is provided

Example:

CreatedDate DATETIME DEFAULT GETDATE()

Simple real-world example

CREATE TABLE Users (
    UserId INT PRIMARY KEY,
    Email VARCHAR(100) UNIQUE NOT NULL,
    Age INT CHECK (Age >= 18),
    CreatedDate DATETIME DEFAULT GETDATE()
);

Here:

• UserId must be unique
• Email cannot be duplicated or empty
• Age must be 18 or above
• CreatedDate auto-fills if not provided

In one line:

Database constraints enforce rules to keep your data correct and trustworthy.