Obrigado @wchiquito por me apontar a direção certa. Eu resolvi o problema. Aqui está como eu fiz isso.
mysql> select * from t1;
+----------------------------------------+------+
| data | id |
+----------------------------------------+------+
| {"key1": "value1", "key2": "VALUE2"} | 1 |
| {"key2": "VALUE2"} | 2 |
| {"key2": "VALUE2"} | 1 |
| {"a": "x", "b": "y", "key2": "VALUE2"} | 1 |
+----------------------------------------+------+
4 rows in set (0.00 sec)
mysql> update t1 set data = JSON_SET(data, "$.key2", "I am ID2") where id = 2;
Query OK, 1 row affected (0.04 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t1;
+----------------------------------------+------+
| data | id |
+----------------------------------------+------+
| {"key1": "value1", "key2": "VALUE2"} | 1 |
| {"key2": "I am ID2"} | 2 |
| {"key2": "VALUE2"} | 1 |
| {"a": "x", "b": "y", "key2": "VALUE2"} | 1 |
+----------------------------------------+------+
4 rows in set (0.00 sec)
mysql> update t1 set data = JSON_SET(data, "$.key3", "I am ID3") where id = 2;
Query OK, 1 row affected (0.07 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from t1;
+------------------------------------------+------+
| data | id |
+------------------------------------------+------+
| {"key1": "value1", "key2": "VALUE2"} | 1 |
| {"key2": "I am ID2", "key3": "I am ID3"} | 2 |
| {"key2": "VALUE2"} | 1 |
| {"a": "x", "b": "y", "key2": "VALUE2"} | 1 |
+------------------------------------------+------+
4 rows in set (0.00 sec)
EDITAR :Se você quiser adicionar um array, use
JSON_ARRAY Como update t1 set data = JSON_SET(data, "$.key4", JSON_ARRAY('Hello','World!')) where id = 2;
