sql

SQL 

What is SQL?

SQL (Structured Query Language) is a standard programming language used to interact with relational databases. It is used to store, retrieve, update, and delete data. 

SQL is also used to create and modify database structures such as tables, views, and indexes.
Example: 
SELECT * FROM Employees; 
This query retrieves all the records from the Employees table.

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What is a Database?

A database is an organized collection of data that is stored and managed electronically. 

It allows users to efficiently store, retrieve, update, and manage data. 

Databases are used to handle large amounts of information in various applications such as websites, business systems, and applications.

Example: A customer database in an e-commerce website may store customer details like name, email, contact number, and purchase history.

1. Create a Database
2. Create Student table
3. Insert Data into Student Table
4. Select/ Retrieve data from table
5. Where (Filter record based on condition) 
6. DISTINCT (Display Unique Record from a table)

Comprehensive

• DDL (Data Definition Language) – Defines the structure of the database. CREATE, ALTER, DROP, TRUNCATE, RENAME.
• DML (Data Manipulation Language) – Manages data stored in the database. INSERT, UPDATE, DELETE.
• DQL (Data Query Language) – Retrieves data from the database. SELECT
• DCL (Data Control Language) – Controls access to the data. GRANT, REVOKE
• TCL (Transaction Control Language) – Manages transactions in the database. COMMIT, ROLLBACK, SAVEPOINT

DDL	DML	DQL	TCL	DCL
CREATE	INSERT	SELECT	COMMIT	GRANT
ALTER	UPDATE		ROLLBACK	REVOKE
DROP	DELETE		SAVE POINT
TRUNCATE
RENAME

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What is Primary Key?

A Primary Key is a column or a combination of columns in a table that uniquely identifies

each row in that table. It does not allow NULL values and must always contain unique

values.

Key Features: 

• Uniquely identifies each record 
• Cannot have duplicate values
• Cannot contain NULL values
• Only one primary key is allowed per table

Example:

This is a SQL Data Definition Language (DDL) statement. 

CREATE TABLE Employee (                 EmployeeID INT PRIMARY KEY,                  Name VARCHAR (50),                  Age INT );

Here, EmployeeID is the primary key that uniquely identifies each employee.

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What is Foreign Key?

A Foreign Key is a column or combination of columns in one table that refers to the Primary Key in another table. 

It is used to create a relationship between two tables and enforce referential integrity. 

Key Features:

• Establishes a relationship between two tables
• Can contain duplicate values
• Can accept NULL values 
• Helps maintain data consistency

Example

Parent table

CREATE TABLE Department (                              DeptID INT PRIMARY KEY,                             DeptName VARCHAR (50)     );

Child Table with Foreign Key

CREATE TABLE Employee (                 EmployeeID INT PRIMARY KEY,                  Name VARCHAR (50),                  Age INT                 DeptID INT,                 FOREIGN KEY (DeptID) REFERENCES Department (DeptID)     );

UNIQUE Key — simple and precise explanation

A UNIQUE key is a database rule (constraint) that does not allow duplicate values in a column (or a group of columns).

What it does

• Every non-NULL value in the column must be different

• It prevents duplicates

• It allows NULL (usually only one NULL, depending on the database)

Key points (no confusion)

• Ensures uniqueness of data

• NULL is allowed (unlike PRIMARY KEY)

• You can have multiple UNIQUE keys in one table

• Used to protect important data like email, phone number, username

Example

CREATE TABLE Users (                   UserID INT PRIMARY KEY,                   Email VARCHAR(100) UNIQUE     );