MySQL 8.0

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CREATE TABLE `item` ( 
                     `id`             INT(11)     NOT NULL, 
                     `name`           VARCHAR(20) NOT NULL, 
                     `characteristic` JSON        NOT NULL 
                    )
INSERT INTO `item` (`id`, `name`, `characteristic`) VALUES
(3, 'Mouse', '[{\"name\": \"length\", \"value\": 6}, 
               {\"name\": \"width\" , \"value\": 3}, 
               {\"name\": \"height\", \"value\": 2}]'),
(4, 'Car'  , '[{\"name\": \"length\", \"value\":11}, 
               {\"name\": \"width\" , \"value\": 7}, 
               {\"name\": \"height\", \"value\": 8}]');
WITH RECURSIVE cte AS 
(
  SELECT 1 AS n
  UNION ALL
  SELECT n + 1 AS value
    FROM cte
   WHERE cte.n < (SELECT MAX(JSON_LENGTH(`characteristic`)) FROM `item` )
)
SELECT i.*
  FROM cte
  JOIN `item` AS i
 WHERE JSON_EXTRACT(`characteristic`,CONCAT('$[',n-1,'].value')) > 10
   AND JSON_EXTRACT(`characteristic`,CONCAT('$[',n-1,'].name')) = "length"


id
name
characteristic


4
Car
[{"name": "length", "value": 11}, {"name": "width", "value": 7}, {"name": "height", "value": 8}]


SELECT  i.*
  FROM `item` AS i
  JOIN (SELECT @i := @i + 1 AS n
          FROM `item` AS i
          JOIN (SELECT @i := -1) AS iter
          LEFT JOIN  information_schema.tables AS t
            ON @i < JSON_LENGTH(`characteristic`) - 1) AS t
 WHERE JSON_EXTRACT(`characteristic`,CONCAT('$[',n,'].value')) > 10
   AND JSON_EXTRACT(`characteristic`,CONCAT('$[',n,'].name')) = "length"            


id
name
characteristic


4
Car
[{"name": "length", "value": 11}, {"name": "width", "value": 7}, {"name": "height", "value": 8}]