MySQL 8.0

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CREATE tABLE employee(id_emp int Primary Key, employee_name varchar(50))
INSERT INTO employee VALUES (1,'Emp0'),(2,'Emp1'),(3,'Emp2'),(4,'Emp3'),(5,'Emp4'),(6,'Emp5'),(7,'Emp6')

Records: 7  Duplicates: 0  Warnings: 0


CREATE TABLE position (id_pos int PRIMARY KEY, position_name varchar(50))
INSERT INTO position VALUES (1,'cashier'), (2,'supervisor')

Records: 2  Duplicates: 0  Warnings: 0


CREATE TABLE Pos_EMP(emp_id int, pos_id int, prio int)
INSERT INTO Pos_EMP VALUES (1,1,1),(2,1,1),(3,1,1),(4,1,2),(4,2,1),(5,1,2),(5,2,1),(6,1,1),(6,2,2),(7,2,1)

Records: 10  Duplicates: 0  Warnings: 0


WITH FIRST_Supervisor as (SELECT id_emp,  employee_name, position_name 
  FROM employee e INNER JOIN Pos_EMP pe ON e.id_emp = pe.emp_id
INNER JOIN position p ON p.id_pos = pe.pos_id
WHERE pos_id = 2 ORDER BY prio, RAND() LIMIT 1)
  SELECT employee_name, position_name  FROM FIRST_Supervisor
  UNION ALL 
(SELECT  employee_name, position_name 
  FROM employee e INNER JOIN Pos_EMP pe ON e.id_emp = pe.emp_id
INNER JOIN position p ON p.id_pos = pe.pos_id
WHERE pos_id = 1 AND emp_id NOT IN ( SELECT id_emp FROM FIRST_Supervisor)  ORDER BY prio, RAND() LIMIT 2)


employee_name
position_name


Emp4
supervisor

Emp5
cashier

Emp2
cashier