{
  fields: [
    {
      key: 'age',
      value: 20
    },
    {
      key: 'years_employed',
      value: 10
    }
  ]
}

.find({fields: {$elemMatch: {key:'age', value:20}}})

.find({fields: {$elemMatch: {key:'age', value:10}}})

{
  fields: [
    {
      key: 'age',
      value: 20
    },
    {
      key: 'years_employed',
      value: 10
    }
  ]
}

SELECT * FROM `data_table` WHERE JSON_CONTAINS(`data`, '{"key":"age","value":20}', "$.fields")