Querying

✅ What is an SQL Query?

SQL (Structured Query Language) is a standard programming language used to communicate with relational databases. SQL Queries are commands used to retrieve, insert, update, delete, and manipulate data stored in relational databases such as MySQL, PostgreSQL, SQL Server, Oracle, SQLite, etc.

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🎯 Types of SQL Queries

Query Type	Description	Example Keyword
Data Query Language (DQL)	Retrieve data from databases	SELECT
Data Manipulation Language (DML)	Manage data (Insert, Update, Delete)	INSERT, UPDATE, DELETE
Data Definition Language (DDL)	Define and modify database structure	CREATE, ALTER, DROP
Data Control Language (DCL)	Control access to data	GRANT, REVOKE
Transaction Control (TCL)	Manage transactions	COMMIT, ROLLBACK

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📊 Basic SQL Query Examples

1. SELECT (Retrieve Data)

sqlCopyEditSELECT first_name, last_name FROM employees;

• Retrieves first_name and last_name from employees table.

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2. WHERE Clause (Filter Data)

sqlCopyEditSELECT * FROM employees WHERE department = 'Sales';

• Retrieves all employees working in the Sales department.

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3. INSERT (Add Data)

sqlCopyEditINSERT INTO employees (first_name, last_name, department) 
VALUES ('John', 'Doe', 'Marketing');

• Inserts a new employee record.

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4. UPDATE (Modify Data)

sqlCopyEditUPDATE employees 
SET department = 'Sales' 
WHERE employee_id = 101;

• Updates the department of employee with ID 101.

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5. DELETE (Remove Data)

sqlCopyEditDELETE FROM employees 
WHERE employee_id = 101;

• Deletes employee record with ID 101.

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🔑 Advanced SQL Query Examples

6. JOIN (Combine Tables)

sqlCopyEditSELECT employees.first_name, departments.department_name
FROM employees
JOIN departments ON employees.department_id = departments.id;

• Retrieves employee names with their department names using a JOIN.

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7. GROUP BY (Aggregate Data)

sqlCopyEditSELECT department, COUNT(*) AS total_employees
FROM employees
GROUP BY department;

• Shows the number of employees in each department.

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8. ORDER BY (Sort Results)

sqlCopyEditSELECT * FROM employees
ORDER BY last_name ASC;

• Lists employees sorted by last name alphabetically.

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9. LIMIT (Restrict Rows Returned)

sqlCopyEditSELECT * FROM employees
LIMIT 5;

• Shows only the first 5 employees.

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10. LIKE (Pattern Matching)

sqlCopyEditSELECT * FROM employees
WHERE first_name LIKE 'J%';

• Finds employees whose first names start with 'J'.

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⚙️ Database Examples to Practice SQL Queries:

Database Name	Description
employees	Employee records (HR data)
customers	Customer data for sales or support systems
orders	E-commerce orders and transactions
products	Product catalog
departments	Organizational units

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🌐 Popular SQL Databases:

Database	Type	Notes
MySQL	Open-source relational DB	Widely used, part of LAMP stack
PostgreSQL	Advanced open-source relational	ACID-compliant, powerful extensions
SQL Server	Microsoft relational DB	Enterprise-level, .NET friendly
SQLite	Embedded relational DB	Lightweight, great for apps/devices
Oracle Database	Commercial relational DB	High performance, enterprise-grade

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🚀 Simple Use Case Scenario:

💡 Example: Get names of employees from "Sales" department sorted by last name.

sqlCopyEditSELECT first_name, last_name
FROM employees
WHERE department = 'Sales'
ORDER BY last_name;

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✅ Summary Table:

Task	SQL Command Example
Retrieve all data	SELECT * FROM table_name;
Retrieve specific columns	SELECT col1, col2 FROM table_name;
Filter records	SELECT * FROM table WHERE condition;
Insert new record	INSERT INTO table (col1, col2) VALUES (val1, val2);
Update existing record	UPDATE table SET col1 = val1 WHERE condition;
Delete record	DELETE FROM table WHERE condition;
Sort results	`ORDER BY column ASC
Aggregate data	GROUP BY column;
Join tables	JOIN, LEFT JOIN, RIGHT JOIN