可以嘗試使用 JSON_EXTRACT 與 JSON_LENGTH 達到你要的結果
範例:
https://stackoverflow.com/a/51035417/9151543
https://stackoverflow.com/a/54673918/9151543

參考這篇

改出這個 db fiddle
看看合不合用

CREATE TABLE TableDate (
  month INT(20),
  name VARCHAR(100));

INSERT INTO TableDate VALUES
('01','[" 99/01/04", " 99/01/05", " 99/01/06", " 99/01/07"]'),
('02','[" 99/02/04", " 99/02/05", " 99/02/06", " 99/02/07"]'),
('03','[" 99/03/04", " 99/03/05", " 99/03/06", " 99/03/07"]');

CREATE TABLE numbers (
  n INT(20) PRIMARY KEY);

INSERT INTO numbers VALUES 
('01'),('02'),('03'),('04'),('05'),('06'),
('07'),('08'),('09'),('10'),('11'),('12');

SELECT
  TableDate.month,
  REPLACE(REPLACE(SUBSTRING_INDEX(SUBSTRING_INDEX(TableDate.name, '", " ', numbers.n), '", " ', -1),'[" ',''),'"]','') date
FROM
  numbers INNER JOIN TableDate
  ON CHAR_LENGTH(TableDate.name)
     -CHAR_LENGTH(REPLACE(TableDate.name, ',', ''))>=numbers.n-1
ORDER BY
  month, n

CREATE TABLE TableDate(
month int(20),	
json varchar(500)
)

INSERT TableDate (month,json)
VALUES 
('01','[" 99/01/04", " 99/01/05", " 99/01/06", " 99/01/07"]'),
('02','[" 99/02/04", " 99/02/05", " 99/02/06", " 99/02/07"]'),
('03','[" 99/03/04", " 99/03/05", " 99/03/06", " 99/03/07"]')

資料格式很整齊，就一招

SELECT month,SUBSTRING(json,4,8) AS F1
FROM TableDate
UNION ALL
SELECT month,SUBSTRING(json,17,8) AS F1
FROM TableDate
UNION ALL
SELECT month,SUBSTRING(json,30,8) AS F1
FROM TableDate
UNION ALL
SELECT month,SUBSTRING(json,43,8) AS F1
FROM TableDate
ORDER BY month,F1

Demo

create table it1210 (
  mon int
, js  json
);
-- 宣告為 json 型態.
-- 原發問者是用 varchar(500)

insert into it1210 values 
('01','[" 99/01/04", " 99/01/05", " 99/01/06", " 99/01/07"]'),
('02','[" 99/02/04", " 99/02/05", " 99/02/06", " 99/02/07"]'),
('03','[" 99/03/04", " 99/03/05", " 99/03/06", " 99/03/07"]');

-- 多增加一筆,資料長度不同的.
insert into it1210 values 
('04','[" 99/04/04", " 99/04/05", " 99/04/06", " 99/04/07", " 99/04/08"]');

select i.mon
     , json_extract(js, concat('$[', c.n, ']')) as dt
  from (select @n := @n + 1 as n
          from (select @n := -1) a
             , (select 1
                  from information_schema.CHARACTER_SETS
                 limit 10) b) c
  join it1210 i
 where c.n < json_length(js)
 order by 1,2;

+------+-------------+
| mon  | dt          |
+------+-------------+
|    1 | " 99/01/04" |
|    1 | " 99/01/05" |
|    1 | " 99/01/06" |
|    1 | " 99/01/07" |
|    2 | " 99/02/04" |
|    2 | " 99/02/05" |
|    2 | " 99/02/06" |
|    2 | " 99/02/07" |
|    3 | " 99/03/04" |
|    3 | " 99/03/05" |
|    3 | " 99/03/06" |
|    3 | " 99/03/07" |
|    4 | " 99/04/04" |
|    4 | " 99/04/05" |
|    4 | " 99/04/06" |
|    4 | " 99/04/07" |
|    4 | " 99/04/08" |
+------+-------------+

8版的方法

select i.mon
     , r.res
  from it1210 i
     , json_table(js, '$[*]' columns (res text path '$')) r
 order by 1, 2;