MySQL 8.0

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CREATE TABLE mytable( myjson json )
INSERT INTO mytable VALUES('{
  "first": "data",
  "second": [1,2,3]
}')
UPDATE mytable 
   SET myjson = JSON_MERGE_PATCH(myjson, '{"third": [4]}') 

Rows matched: 1  Changed: 1  Warnings: 0

SELECT myjson FROM mytable

myjson

7b226669727374223a202264617461222c20227468697264223a205b345d2c20227365636f6e64223a205b312c20322c20335d7d

UPDATE mytable 
   SET myjson = JSON_MERGE_PATCH(myjson, '{"third": [4]}') 

Rows matched: 1  Changed: 0  Warnings: 0

SELECT myjson FROM mytable

myjson

7b226669727374223a202264617461222c20227468697264223a205b345d2c20227365636f6e64223a205b312c20322c20335d7d

SELECT JSON_PRETTY(myjson) FROM mytable

JSON_PRETTY(myjson)

{
  "first": "data",
  "third": [
    4
  ],
  "second": [
    1,
    2,
    3
  ]
}