Aggregation Functions

 Aggregation functions in MySQL are used to perform calculations on multiple values and return a single value. These functions are commonly used with the GROUP BY clause. Here are some of the most commonly used aggregation functions in MySQL:

1. COUNT()
Counts the number of rows that match a specified condition.

SELECT COUNT(*)

FROM students;

2. SUM()
Calculates the sum of a set of values.

SELECT SUM(score)

FROM scores;

3. AVG()
Calculates the average value of a set of values.

SELECT AVG(score)

FROM scores;

4. MIN()
Finds the minimum value in a set of values.

SELECT MIN(score)

FROM scores;

5. MAX()
Finds the maximum value in a set of values.

SELECT MAX(score)

FROM scores;

6. GROUP_CONCAT()
Concatenates values from multiple rows into a single string.

SELECT GROUP_CONCAT(course_name)

FROM courses;

7.VARIANCE()

Returns the variance of a numeric column.
SELECT VARIANCE(salary) FROM employees;


8.STDDEV()

Returns the standard deviation of a numeric column.
SELECT STDDEV(salary) FROM employees;


9.BIT_AND()

Returns the bitwise AND of all bits in a column.
SELECT BIT_AND(some_bitwise_column) FROM employees;

10.BIT_OR()

Returns the bitwise OR of all bits in a column.
SELECT BIT_OR(some_bitwise_column) FROM employees;

11.JSON_ARRAYAGG()

Aggregates values as a JSON array.
SELECT JSON_ARRAYAGG(employee_name) FROM employees;

12.JSON_OBJECTAGG()

Aggregates key-value pairs as a JSON object.
SELECT JSON_OBJECTAGG(employee_id, employee_name) FROM employees;

These functions are typically used in combination with the GROUP BY clause to aggregate data across different groups within a table. 

Example:

We'll have two tables: departments and employees.

Departments Table

CREATE TABLE departments (
department_id INT PRIMARY KEY,
department_name VARCHAR(100)
);

Employees Table

CREATE TABLE employees (

    employee_id INT PRIMARY KEY,

    employee_name VARCHAR(100),

    salary DECIMAL(10, 2),

    department_id INT,

    join_date DATE,

    FOREIGN KEY (department_id) REFERENCES departments(department_id)

);

Sample Data
Inserting Data into Departments Table

INSERT INTO departments (department_id, department_name) VALUES

(1, 'HR'),

(2, 'Engineering'),

(3, 'Marketing');

Inserting Data into Employees Table

INSERT INTO employees (employee_id, employee_name, salary, department_id, join_date) VALUES

(1, 'Alice', 60000, 2, '2020-01-15'),

(2, 'Bob', 50000, 1, '2019-03-20'),

(3, 'Charlie', 70000, 2, '2021-07-30'),

(4, 'David', 55000, 3, '2018-11-12'),

(5, 'Eve', 65000, 2, '2020-05-01');

Aggregation Queries

Total number of employees:

SELECT COUNT(*) AS total_employees FROM employees;

Total salary paid to employees:

SELECT SUM(salary) AS total_salary FROM employees;

Average salary per department:

SELECT department_id, AVG(salary) AS average_salary 

FROM employees 

GROUP BY department_id;

Minimum and maximum salary in the company:

SELECT MIN(salary) AS min_salary, MAX(salary) AS max_salary 

FROM employees;

Concatenated list of employee names per department:

SELECT department_id, GROUP_CONCAT(employee_name) AS employees 

FROM employees 

GROUP BY department_id;

Variance and standard deviation of salaries:SELECT VARIANCE(salary) AS salary_variance, STDDEV(salary) AS salary_stddev 

FROM employees;

Bitwise AND and OR of employee IDs:SELECT BIT_AND(employee_id) AS bit_and_result, BIT_OR(employee_id) AS bit_or_result 

FROM employees;

Aggregated employee names as JSON array per department:

SELECT department_id, JSON_ARRAYAGG(employee_name) AS employee_names 

FROM employees 

GROUP BY department_id;

Aggregated key-value pairs of employee IDs and names as JSON object:

SELECT JSON_OBJECTAGG(employee_id, employee_name) AS employee_id_name_map 

FROM employees;