SQL Constraints

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

👉 In simple words:
Constraints restrict the type of data that can be inserted into a table.

Why Constraints Are Important
Constraints ensure:
   • Data is valid
   • No duplicate values
   • Required fields are not empty
   • Tables maintain proper relationships

Example:
A student table should not allow two students with the same Student_ID.

Types of SQL Constraints
The most common SQL constraints are:
1. NOT NULL
2. UNIQUE
3. PRIMARY KEY
4. FOREIGN KEY
5. CHECK
6. DEFAULT

Simple Example (All Constraints Together)

CREATE TABLE Employees (
    Employee_ID INT PRIMARY KEY,
    Name VARCHAR(50) NOT NULL,
    Email VARCHAR(100) UNIQUE,
    Age INT CHECK (Age >= 18),
    Country VARCHAR(50) DEFAULT 'India'
);

Summary Table

Constraint    |	    Purpose
--------------|----------------------
NOT NULL --------- Prevents empty values
UNIQUE ----------- Ensures all values are different
PRIMARY KEY ------ Unique identifier for table
FOREIGN KEY------- Connects two tables
CHECK	---------  Validates condition
DEFAULT	 ---------- Assigns default value

This table enforces:
   • Unique employee IDs
   • Name cannot be empty
   • Email must be unique
   • Age must be ≥ 18
   • Default country is India


Most important constraints for interviews:
1. PRIMARY KEY
2. FOREIGN KEY
3. UNIQUE
4. NOT NULL

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