• INNER JOIN: Returns rows that have matching values in both tables.

SELECT e.name, e.salary, d.department_name
FROM employees e
INNER JOIN departments d ON e.department = d.department_id;

• LEFT JOIN: Returns all rows from the left table and matched rows from the right table. If no match, returns NULL.

SELECT e.name, e.salary, d.department_name
FROM employees e
LEFT JOIN departments d ON e.department = d.department_id;

• RIGHT JOIN: Returns all rows from the right table and matched rows from the left table. If no match, returns NULL.

SELECT e.name, e.salary, d.department_name
FROM employees e
RIGHT JOIN departments d ON e.department = d.department_id;

• FULL OUTER JOIN: Returns all rows when there is a match in one of the tables.

SELECT e.name, e.salary, d.department_name
FROM employees e
FULL OUTER JOIN departments d ON e.department = d.department_id;

6. Subqueries and Nested Queries

Subqueries are queries embedded inside other queries. They can be used in the SELECT, FROM, and WHERE clauses.

Correlated Subqueries

A correlated subquery references columns from the outer query.

-- Find employees with salaries above the average salary of their department
SELECT name, salary
FROM employees e1
WHERE salary > (SELECT AVG(salary)
FROM employees e2
WHERE e1.department = e2.department);

Using Subqueries in SELECT

You can also use subqueries in the SELECT statement:

SELECT name,
(SELECT AVG(salary) FROM employees) AS avg_salary
FROM employees;

7. Advanced SQL

Window Functions

Window functions perform calculations across a set of table rows related to the current row. They do not collapse rows like GROUP BY.

-- Rank employees by salary within each department
SELECT name, department, salary,
RANK() OVER (PARTITION BY department ORDER BY salary DESC) AS rank
FROM employees;

Common Table Expressions (CTEs)

A CTE is a temporary result set that can be referenced within a SELECT, INSERT, UPDATE, or DELETE statement.

-- Calculate department-wise average salary using a CTE
WITH avg_salary_cte AS (
SELECT department, AVG(salary) AS avg_salary
FROM employees
GROUP BY department
)
SELECT e.name, e.salary, a.avg_salary
FROM employees e
JOIN avg_salary_cte a ON e.department = a.department;

8. Data Transformation and Cleaning

CASE Statements

The CASE statement allows you to perform conditional logic within SQL queries.

-- Categorize employees based on salary
SELECT name,
CASE
WHEN salary < 50000 THEN 'Low'
WHEN salary BETWEEN 50000 AND 100000 THEN 'Medium'
ELSE 'High'
END AS salary_category
FROM employees;

String Functions

SQL offers several functions to manipulate strings:

-- Concatenate first and last names
SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM employees;

-- Trim extra spaces from a string
SELECT TRIM(name) FROM employees;

Date and Time Functions

SQL allows you to work with date and time values:

-- Calculate tenure in days
SELECT name, DATEDIFF(CURDATE(), hire_date) AS days_tenure
FROM employees;

9. Database Management

Indexing

Indexes improve query performance by allowing faster retrieval of rows.

-- Create an index on the department column for faster lookups
CREATE INDEX idx_department ON employees(department);

Views

A view is a virtual table based on the result of a query. It simplifies complex queries by allowing you to reuse the logic.

-- Create a view for high-salary employees
CREATE VIEW high_salary_employees AS
SELECT name, salary
FROM employees
WHERE salary > 100000;

-- Query the view
SELECT * FROM high_salary_employees;

Transactions

A transaction ensures that a series of SQL operations are completed successfully. If any part fails, the entire transaction can be rolled back to maintain data integrity.

-- -- Transaction example
START TRANSACTION;
UPDATE employees SET salary = salary + 5000 WHERE department = 'HR';
DELETE FROM employees WHERE id = 10;
COMMIT; -- Commit the transaction if all

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