How to implement many-to-many relationship with one table I have a table:

CREATE DATABASE IF NOT EXISTS test;
USE test;

CREATE TABLE IF NOT EXISTS emp_list(
id INT UNSIGNED NOT NULL AUTO_INCREMENT primary key,
first_name VARCHAR(30) NOT NULL,
last_name VARCHAR(30) NOT NULL,
job_position  VARCHAR(20) NOT NULL,
salary decimal(15,2) UNSIGNED,
);

INSERT INTO emp_list ( id, first_name, last_name, job_position, salary) VALUES ( null, 'Ivan', 'Noscov', 'Director', 69000);
INSERT INTO emp_list ( id, first_name, last_name, job_position, salary) VALUES ( null, 'Igor', 'Malov', 'Accountant', 54000);
INSERT INTO emp_list ( id, first_name, last_name, job_position, salary) VALUES ( null, 'Ira', 'Solnceva', 'Salesperson', 27000);
INSERT INTO emp_list ( id, first_name, last_name, job_position, salary) VALUES ( null, 'Ivan', 'Noscov', 'Security guard', 15000);
UPDATE employees.emp_list SET last_name = 'Varnaev' WHERE id=4; 
INSERT INTO emp_list ( id, first_name, last_name, job_position, salary) VALUES ( null, 'Natasha', 'Bolshova', 'Salesperson', 27000);


I need update the database schematic so that it is possible to store the information about
subordinates. No restrictions: each employee can have several subordinates (or not have them)
and may have more than one boss (or no boss at all). Request a sample of all
of a certain employee's subordinates.

Need to do this by adding a new column and need to use a foreign key to create relationships
You cannot do this with one additional column You need to create a new table for relations between employees. 

CREATE TABLE IF NOT EXISTS emp_relations(
emp_id1 INT UNSIGNED NOT NULL,
emp_id2 INT UNSIGNED NOT NULL,

FOREIGN KEY (emp_id1 )
      REFERENCES emp_list(id),
FOREIGN KEY (emp_id2 )
      REFERENCES emp_list(id),

UNIQUE KEY(emp_id1 ,emp_id2)  
 );


Link both columns to emp_list.id. Make emp_id1 and emp_id2 together unique.

This way you will have hierarchy and many to many relations.

Mysql - Foreign keys

Mysql - Unique keys