mysql JSON_SET can't insert into column with NULL value(5.7+)
Use COALESCE:
UPDATE testjson SET extra = JSON_SET(COALESCE(extra, '{}'), '$.age', 87);
SET extra = JSON_SET(IFNULL(extra, "{}"), "$.age", 87)
Use COALESCE:
UPDATE testjson SET extra = JSON_SET(COALESCE(extra, '{}'), '$.age', 87);
SET extra = JSON_SET(IFNULL(extra, "{}"), "$.age", 87)