Implementation of removing overlapping time and calculating time difference in MySQL

SELECT TIMESTAMPDIFF(MINUTE, '2021-08-19 14:30:00', '2021-08-19 15:00:00') FROM DUAL;

CREATE TABLE test01 (
  id int(32) unsigned NOT NULL AUTO_INCREMENT,
  start_time datetime NOT NULL,
  end_time datetime NOT NULL,
  PRIMARY KEY (`id`)
) 
 
INSERT INTO test01(id, start_time, end_time) VALUES (1, '2021-08-18 16:27:51', '2021-08-18 17:27:59');
INSERT INTO test01(id, start_time, end_time) VALUES (2, '2021-08-18 17:20:26', '2021-08-18 20:10:37');
INSERT INTO test01(id, start_time, end_time) VALUES (3, '2021-08-18 22:05:57', '2021-08-18 23:55:20'); 

CREATE PROCEDURE sumTime()
BEGIN
    -- Define variable -- Is it the first time to DECLARE is_old int(1) DEFAULT 0;
 
    -- Last data DECLARE old_start_time datetime;
	DECLARE old_end_time datetime;
 
	-- This data DECLARE start_time datetime;
	DECLARE end_time datetime;
 
	-- Return result DECLARE num int(32) DEFAULT 0;
 
	-- Loop end switch DECLARE done int DEFAULT 0;
 
	-- Create a cursor (query database data)
	DECLARE list CURSOR FOR SELECT a.start_time, a.end_time FROM test01 a;
 
    -- Define the last loop and set the loop end switch to 1
	DECLARE CONTINUE HANDLER FOR SQLSTATE '02000' SET done=1;
 
	--Open cursor OPEN list;
 
		-- Open loop posLoop:LOOP
			-- Take the value of the current loop and assign it to the current data variable FETCH list INTO start_time,end_time;
			-- Determine if it is the first time if (is_old = 0) THEN 
 
				SET is_old = 1;
				SET old_start_time = start_time;
				SET old_end_time = end_time;
 
			-- ELSE
				-- Check if it is within the interval if (start_time >= old_start_time AND start_time <= old_end_time) THEN
 
					-- Check if the end time is not in the interval if (end_time < old_start_time OR end_time > old_end_time) THEN
						SET old_end_time = end_time;
				   END IF;
 
				 -- ELSE
 
				   if (start_time < old_start_time ) THEN
 
						SET old_start_time = start_time;
 
					 ELSE
 
						SET num = num + TIMESTAMPDIFF(MINUTE, old_start_time, old_end_time);
						SET old_start_time = start_time;
						SET old_end_time = end_time;
					 END IF;
				 END IF;
			END IF;
			-- Check if it is the last loop IF done=1 THEN 
			    SET num = num + TIMESTAMPDIFF(MINUTE, old_start_time, old_end_time);
			    LEAVE posLoop;
			END IF;
		-- End the loop	
		END LOOP posLoop;
	-- Close the cursor CLOSE list;
	SELECT num;
END;

-- Call the stored procedure call sumTime(); 

-- Delete the stored procedure drop procedure if exists sumTime;