试试这个。在mysql 8上进行了测试。我把它分解成几个步骤，这样你就可以一个接一个地测试每个cte，看看它是如何结合在一起的。

表

create table location (id int, name varchar(10), neighbors json);
insert into location (id, name) values
(1, 'loc1'), (2, 'loc2'), (3, 'loc3'), (4, 'loc4'), (5, 'loc5');

create table nearby (place int, nextdoor int, distance varchar(10), mygroup varchar(10));
insert into nearby values
(1, 2, '500m', 'g1'), (1, 3, '900m', 'g1'),
(2, 1, '500m', 'g3'), (2, 3, '100m', 'g4'), (2, 4, '80m', 'g4');

更新查询

with flatten_neighbor as (
    select nn.place, nn.name, l.name as nextdoorname, nn.distance, nn.mygroup
    from (
        select l.id, l.name, n.place, n.nextdoor, n.distance, n.mygroup
        from location l
        join nearby n on n.place = l.id
    ) nn
    join location l on l.id = nn.nextdoor
),
obj_by_place as (
    select *, json_object('name', nextdoorname, 'distance', distance) as jo 
    from flatten_neighbor
),
concat_obj as (
    select place, mygroup, concat('[', group_concat(jo), ']') as jason
    from obj_by_place group by place, mygroup
),
final as (
    select place, json_objectagg(mygroup, jason) stuff
    from concat_obj group by place
)
update location l join final f on l.id = f.place set l.neighbors = f.stuff;

结果

id | name | neighbors                                                                                                                                                   
-: | :--- | :-----------------------------------------------------------------------------------------------------------------------------------------------------------
 1 | loc1 | {"g1": "[{\"name\": \"loc2\", \"distance\": \"500m\"},{\"name\": \"loc3\", \"distance\": \"900m\"}]"}                                                       
 2 | loc2 | {"g3": "[{\"name\": \"loc1\", \"distance\": \"500m\"}]", "g4": "[{\"name\": \"loc3\", \"distance\": \"100m\"},{\"name\": \"loc4\", \"distance\": \"80m\"}]"}
 3 | loc3 | null                                                                                                                                                        
 4 | loc4 | null                                                                                                                                                        
 5 | loc5 | null

示例

https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=789fb79d10b745414ca7270a5a6ef004
另一个例子是更新后的json提取。
https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=fe651372ca754cc05eab842d6fced9a3