MySQL 8.0

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SELECT VERSION();


VERSION()


8.0.26


SET @`default_values` := JSON_OBJECT(
  'a', 1,
  'b', 2,
  'c', 3,
  'd', 4
);
SELECT
  JSON_PRETTY(
    JSON_MERGE_PATCH(
      @`default_values`,
      JSON_OBJECT(
        'a', 5,
        'e', 6
      )
    )
  );


JSON_PRETTY(
    JSON_MERGE_PATCH(
      @`default_values`,
      JSON_OBJECT(
        'a', 5,
        'e', 6
      )
    )
  )


{
  "a": 5,
  "b": 2,
  "c": 3,
  "d": 4,
  "e": 6
}