Running the loop stops after changing the Counter Variable - mysql

Question

When I run the following code, it always returns a NULL value for the two variables inside the loop.
SET #c = '';
SET #i = 4;
REPEAT
SET #c = CONCAT(
' -> ',
(
SELECT name
FROM c
WHERE id = #i
),
#c
);
SET #i = (
SELECT pid
FROM c
WHERE id = #i
);
UNTIL TRIM(COALESCE(#i, '')) = ''
END REPEAT;
The loop must be run at least 4 times; But in the second run, the run of the loop ends.
The value is set for variables in the first time that the loop is executed; But in the second run of the loop, the value of both changes to NULL and the loop stops. However, inside table c there are related values for the execution of the loop.
The problem occurs when the value of the variable #i in the loop
changes.
For example, the first time the loop is executed, the value of #i becomes 3, and there is an id with a value of 3 in table c in addition to the initial value of 4.
Structure of Table c
CREATE TABLE c (
id int UNSIGNED NOT NULL AUTO_INCREMENT,
name varchar(50) NOT NULL,
pid int UNSIGNED DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE = INNODB,
AUTO_INCREMENT = 1,
CHARACTER SET utf8mb4,
COLLATE utf8mb4_unicode_ci;
ALTER TABLE c
ADD CONSTRAINT c_pid_foreign FOREIGN KEY (pid) REFERENCES c (id);
INSERT INTO c(id, name, pid)
VALUES (1, '1st', NULL),
(2, '2nd', 1),
(3, '3rd', 2),
(4, '4th', 3);
Thank you in advance for the good cooperation of the people to solve this problem.

Answer 1

I was finally able to solve my question after about more than a day; Of course, I did not understand the cause of this problem and I do not know why MySQL changes the return value of the result of the SELECT statement to NULL when using the #i variable after changing its value inside the loop?!!!
The solution is as follows:
SET #d = '';
SET #j = 4;
REPEAT
SELECT #c := name
FROM c
WHERE id = #j;
SET #d = CONCAT(' -> ', #c, #d);
SELECT #i := pid
FROM c
WHERE id = #j;
SET #j = #i;
UNTIL TRIM(COALESCE(#j, '')) = ''
END REPEAT;
Here are the best solution to this problem:
SET #d = '';
SET #j = 4;
REPEAT
SELECT name INTO #c
FROM c
WHERE id = #j;
SET #d = CONCAT(' -> ', #c, #d);
SELECT pid INTO #i
FROM c
WHERE id = #j;
SET #j = #i;
UNTIL TRIM(COALESCE(#j, '')) = ''
END REPEAT;
I hope it is useful for others as well.