MariaDB 10.3

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CREATE TABLE t( features VARCHAR(1500) )
INSERT INTO t VALUES
('{
  "id": 9,
  "details": [
    {
      "amount": 100,
      "flag": true,
      "fieldA": "abcd"
    },
    {
      "amount": 101,
      "flag": false,
      "fieldB": "bcde"
    },
    {
      "amount": 103,
      "flag": true,
      "fieldA": "abcd"
    }
  ]
}'),
('{
  "id": 10,
  "details": [
    {
      "amount": 110,
      "flag": false,
      "fieldA": "abcd"
    },
    {
      "amount": 102,
      "flag": true,
      "fieldB": "bcde"
    }
  ]
}')

Records: 2  Duplicates: 0  Warnings: 0


SELECT * FROM t


features


{
  "id": 9,
  "details": [
    {
      "amount": 100,
      "flag": true,
      "fieldA": "abcd"
    },
    {
      "amount": 101,
      "flag": false,
      "fieldB": "bcde"
    },
    {
      "amount": 103,
      "flag": true,
      "fieldA": "abcd"
    }
  ]
}

{
  "id": 10,
  "details": [
    {
      "amount": 110,
      "flag": false,
      "fieldA": "abcd"
    },
    {
      "amount": 102,
      "flag": true,
      "fieldB": "bcde"
    }
  ]
}


WITH RECURSIVE cte AS
(
 SELECT 0 i
 UNION ALL
 SELECT i + 1 i
   FROM cte
  WHERE i + 1 <= ( SELECT MAX(JSON_LENGTH(features, '$.details')) FROM t ) 
)
SELECT  SUM( CASE 
            WHEN JSON_EXTRACT(features, CONCAT('$.details[',i,'].flag')) = 'true' THEN
                 CAST(JSON_EXTRACT(features, CONCAT('$.details[',i,'].amount')) AS DOUBLE)
             END
          ) AS sum_amount 
  FROM cte,
       t


sum_amount


305