/SQL-Complete-Reference-Basic-Intermediate-Advanced

Commonly Used SQL Functions

SQL functions

This repository contains examples of the main categories of SQL functions, including aggregation functions, string functions, date and time functions, comparison functions, logical functions, conversion functions, and system functions.

Content

Aggregation functions

Aggregation functions calculate summary values ​​from data sets.

Example:

SELECT SUM(salary) FROM employees;

Other aggregation functions include:

SELECT AVG(price) FROM products;
SELECT COUNT(DISTINCT customer_id) FROM orders;
SELECT MAX(score) FROM exams;
SELECT MIN(age) FROM users;

String Functions

String functions manipulate text strings.

Example:

SELECT UPPER(name) FROM customers;

Other string functions include:

SELECT CONCAT(first_name, ' ', last_name) AS full_name FROM employees;
SELECT SUBSTRING(phone_number, 1, 3) AS area_code FROM contacts;
SELECT LENGTH(address) FROM locations;
SELECT TRIM(LEADING ' ' FROM username) FROM accounts;

Date and Time Functions

These functions work with date and time values.

Example:

SELECT DATE(datetime_column) FROM events;

Other date and time functions include:

SELECT YEAR(hire_date) FROM employees;
SELECT MONTH(birth_date) FROM students;
SELECT CURRENT_TIMESTAMP();
SELECT DATEDIFF(end_date, start_date) AS duration FROM projects;

Comparison Functions

Comparison functions compare values and return boolean values (TRUE or FALSE).

Example:

SELECT * FROM users WHERE age = 30;

Other comparison functions include:

SELECT * FROM products WHERE price > 100;
SELECT * FROM orders WHERE status <> 'Completed';
SELECT * FROM employees WHERE hire_date BETWEEN '2020-01-01' AND '2020-12-31';
SELECT * FROM customers WHERE name LIKE 'J%';

Logical Functions

Logical functions perform logical operations on boolean values.

Example:

SELECT * FROM customers WHERE age > 18 AND city = 'New York';

Other logical functions include:

SELECT * FROM orders WHERE status = 'Pending' OR status = 'Processing';
SELECT * FROM students WHERE NOT graduated;
SELECT * FROM products WHERE category IN ('Electronics', 'Clothing');
SELECT * FROM users WHERE email IS NULL;

Conversion Functions

Conversion functions convert values from one data type to another.

Example:

SELECT CAST(quantity AS VARCHAR) FROM inventory;

Other conversion functions include:

SELECT CONVERT(price, DECIMAL(10,2)) FROM products;
SELECT CAST(order_date AS DATE) FROM orders;
SELECT CAST(is_active AS INTEGER) FROM users;
SELECT CAST(CONCAT(first_name, ' ', last_name) AS CHAR(50)) FROM employees;

System Functions

System functions provide information about the system or database.

Example:

SELECT USER();

Other system functions include:

SELECT DATABASE();
SELECT VERSION();
SELECT CURRENT_USER();
SELECT LAST_INSERT_ID();

Window Functions

Window functions perform calculations across a set of rows that are related to the current row.

Example:

SELECT employee_id, department, salary, AVG(salary) OVER (PARTITION BY department) AS avg_department_salary FROM employees;

Other window functions include:

SELECT product_id, category, price, RANK() OVER (PARTITION BY category ORDER BY price DESC) AS price_rank FROM products;
SELECT order_id, customer_id, order_date, LAG(order_date) OVER (PARTITION BY customer_id ORDER BY order_date) AS previous_order_date FROM orders;
SELECT student_id, subject, score, DENSE_RANK() OVER (PARTITION BY subject ORDER BY score DESC) AS subject_rank FROM exam_results;
SELECT sales_rep, order_date, sales_amount, SUM(sales_amount) OVER (PARTITION BY sales_rep ORDER BY order_date) AS running_total FROM sales;