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1. What is a Database?

A Database is a structured collection of data that is organized, stored, and managed so it can be easily accessed, updated, and managed.

Key Points:

• Stores information in a structured way (tables, rows, columns).
• Enables efficient data retrieval and manipulation.
• Used in almost every software application: websites, mobile apps, ERP systems, etc.

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2. Types of Databases

Type	Description	Examples
Relational Database (RDBMS)	Data is stored in tables with rows and columns, relationships between tables are maintained	MySQL, PostgreSQL, Oracle, SQL Server
NoSQL Database	Non-tabular, flexible schema, good for unstructured data	MongoDB, Cassandra, Redis
In-Memory Database	Stores data in memory for faster access	Redis, Memcached
Cloud Database	Hosted in the cloud, scalable	AWS RDS, Google Cloud SQL, Azure SQL Database

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3. What is SQL?

SQL (Structured Query Language) is a standard language used to communicate with relational databases.

Purpose:

• Retrieve data (SELECT)
• Insert data (INSERT)
• Update data (UPDATE)
• Delete data (DELETE)
• Create and modify database objects (CREATE, ALTER, DROP)

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4. Basic SQL Commands

A. Data Definition Language (DDL)

Used to create and modify database structure.

Command	Description	Example
CREATE DATABASE	Create a new database	CREATE DATABASE mydb;
CREATE TABLE	Create a new table	CREATE TABLE users(id INT, name VARCHAR(50));
ALTER TABLE	Modify table structure	ALTER TABLE users ADD email VARCHAR(50);
DROP TABLE	Delete a table	DROP TABLE users;

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B. Data Manipulation Language (DML)

Used to manipulate data in tables.

Command	Description	Example
INSERT INTO	Insert new records	INSERT INTO users(id, name) VALUES(1, 'John');
SELECT	Retrieve records	SELECT * FROM users;
UPDATE	Update existing records	UPDATE users SET name='Mike' WHERE id=1;
DELETE	Delete records	DELETE FROM users WHERE id=1;

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C. Data Control Language (DCL)

Used to control access to the database.

Command	Description	Example
GRANT	Give permissions	GRANT SELECT ON users TO 'testuser';
REVOKE	Remove permissions	REVOKE SELECT ON users FROM 'testuser';

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D. Transaction Control Language (TCL)

Used to manage transactions.

Command	Description
COMMIT	Save all changes
ROLLBACK	Undo changes
SAVEPOINT	Set a point to rollback to

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5. Database Concepts

1. Table – Collection of related data in rows and columns.
2. Row / Record – Single entry in a table.
3. Column / Field – Attribute of a record.
4. Primary Key (PK) – Unique identifier for each record.
5. Foreign Key (FK) – Field linking one table to another.
6. Index – Improves search performance.
7. Normalization – Organizing data to reduce redundancy.
8. Relationships:
   • One-to-One – One record in table A corresponds to one record in table B.
   • One-to-Many – One record in table A corresponds to multiple records in table B.
   • Many-to-Many – Many records in table A correspond to many in table B (via junction table).

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6. SQL Joins

Used to combine data from multiple tables.

Join Type	Description
INNER JOIN	Returns records with matching values in both tables
LEFT JOIN (or LEFT OUTER)	Returns all records from left table, matched records from right table
RIGHT JOIN (or RIGHT OUTER)	Returns all records from right table, matched records from left table
FULL OUTER JOIN	Returns all records when there is a match in one of the tables
CROSS JOIN	Returns Cartesian product (all combinations)

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7. Best Practices for SQL & Databases

1. Always use primary keys for tables.
2. Avoid redundant data; normalize tables.
3. Use indexes to improve performance.
4. Use transactions for critical operations.
5. Write clear and optimized queries to reduce load.
6. Backup databases regularly.
7. Use descriptive names for tables and columns.

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8. Summary

• Database stores structured information efficiently.
• SQL is used to create, read, update, and delete data in relational databases.
• Basic SQL commands include DDL, DML, DCL, and TCL.
• Key concepts include tables, keys, relationships, joins, and normalization.
• Good practices improve performance, scalability, and reliability.

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