Primary Key | Foreign Key | Unique Key | Null Not | Check Constraint | Integrity |Constraint in SQL

SQL Constraints – Complete Explanation
🔹 What are Constraints?

Constraints are rules applied to table columns in SQL to ensure data accuracy, consistency, and integrity.

👉 They prevent invalid data from being entered into the database.

🔹 Types of SQL Constraints
✅ 1. NOT NULL Constraint
Ensures a column cannot have NULL (empty) values

Example:

CREATE TABLE Student (
ID INT NOT NULL,
Name VARCHAR(50) NOT NULL
);
👉 Every record must have ID and Name

✅ 2. UNIQUE Constraint
Ensures all values in a column are different

Example:

CREATE TABLE Student (
Email VARCHAR(100) UNIQUE
);
👉 No duplicate email allowed

✅ 3. PRIMARY KEY Constraint
Uniquely identifies each record
Cannot contain NULL values
Only one primary key per table

Example:

CREATE TABLE Student (
ID INT PRIMARY KEY,
Name VARCHAR(50)
);
👉 ID will be unique for every student

✅ 4. FOREIGN KEY Constraint
Creates a relationship between two tables
Refers to a PRIMARY KEY in another table

Example:

CREATE TABLE Orders (
OrderID INT,
StudentID INT,
FOREIGN KEY (StudentID) REFERENCES Student(ID)
);
👉 Ensures StudentID exists in Student table

✅ 5. CHECK Constraint
Ensures values meet a specific condition

Example:

CREATE TABLE Student (
Age INT CHECK (Age = 18)
);
👉 Age must be 18 or above

✅ 6. DEFAULT Constraint
Assigns a default value if no value is provided

Example:

CREATE TABLE Student (
Country VARCHAR(50) DEFAULT 'India'
);
👉 If no value is given → "India" is stored

🎯 Why Constraints are Important?
Maintain data integrity
Avoid duplicate or invalid data
Improve database reliability
Help in data relationships
🚀 Summary
NOT NULL → No empty values
UNIQUE → No duplicate values
PRIMARY KEY → Unique identifier
FOREIGN KEY → Table relationship
CHECK → Condition validation
DEFAULT → Default value

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