#json解析函数       
DELIMITER $$
DROP FUNCTION IF EXISTS `json_extract_c`$$
CREATE FUNCTION `json_extract_c`(
details TEXT,
required_field VARCHAR (255)
) RETURNS TEXT CHARSET latin1
BEGIN
SET details = SUBSTRING_INDEX(details, "{", -1);
SET details = SUBSTRING_INDEX(details, "}", 1);
RETURN TRIM(
    BOTH '"' FROM SUBSTRING_INDEX(
        SUBSTRING_INDEX(
            SUBSTRING_INDEX(
                details,
                CONCAT(
'"',
                    SUBSTRING_INDEX(required_field,'$.', -1),
'":'
                ),
-1
            ),
',"',
1
        ),
':',
-1
    )
) ;
END$$
DELIMITER ;
example：
select json_extract_c(json, "$.totalDays"), json from item limit 100;

自测
CREATE TABLE `json_test` (
  `id` int(11) DEFAULT NULL,
  `person_desc` text COLLATE utf8mb4_unicode_ci
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;


insert INTO json_test values(2,'{"firstName":"Sergei","lastName":"Rachmaninoff","instrument":"piano"}');
SELECT id,json_extract_c(person_desc,'$.lastName') as "keys" FROM json_test;

 改进，再找不到key，返回''值，之前的是在找不到的情况下，查找到第一的值。

CREATE DEFINER=`zhangfen`@`%` FUNCTION `json_extract_c`(
details TEXT,
required_field VARCHAR (255)
) RETURNS text CHARSET latin1
BEGIN
SET details = SUBSTRING_INDEX(details, "{", -1);
SET details = SUBSTRING_INDEX(details, "}", 1);
RETURN TRIM(
    BOTH '"' FROM SUBSTRING_INDEX(
        SUBSTRING_INDEX(
            SUBSTRING_INDEX(
                CONCAT('"":"",',details),
                CONCAT(
                    '"',
                    SUBSTRING_INDEX(required_field,'$.', -1),
                    '":'
                ),
                -1
            ),
            ',"',
            1
        ),
        ':',
        -1
    )
) ;
END