MySQL 8.0

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CREATE TABLE table1
SELECT 1 id,	CAST('{"key3": "value3"}' AS JSON) data UNION ALL
SELECT 2,	CAST('{"key5": "value5"}' AS JSON);
SELECT id, CAST(data AS CHAR) data FROM table1;

Records: 2  Duplicates: 0  Warnings: 0

id
data

1
{"key3": "value3"}

2
{"key5": "value5"}

CREATE TABLE table2
SELECT 1 id,	'key10' `key`,	'value10' value UNION ALL
SELECT 2,	'key13',	'value13';
SELECT * FROM table2;

Records: 2  Duplicates: 0  Warnings: 0

id
key
value

1
key10
value10

2
key13
value13

UPDATE table1 
JOIN table2 USING (id)
SET table1.data = JSON_MERGE_PATCH(table1.data, JSON_OBJECT(table2.`key`, table2.value));

Rows matched: 2  Changed: 2  Warnings: 0

SELECT id, CAST(data AS CHAR) data FROM table1;

id
data

1
{"key3": "value3", "key10": "value10"}

2
{"key5": "value5", "key13": "value13"}

id	data
1	{"key3": "value3", "key10": "value10"}
2	{"key5": "value5", "key13": "value13"}