MySQL 8.0

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create table user (id int, name varchar(50));
create table sale (id int, user int, item varchar(50))
insert into user values(1, 'User 1')
insert into user values(2, 'User 2')
insert into sale values (1, 1, 't-shirt');
insert into sale values (2, 1, 'jeans');
insert into sale values (3, 2, 'sweatpants');
insert into sale values (4, 2, 'gloves');
SELECT
  JSON_PRETTY(JSON_ARRAYAGG(JSON_OBJECT('id', u.id, 'name', u.name, 'sales', s.sales)))
FROM
    user u
    left join (SELECT user, JSON_ARRAYAGG(JSON_OBJECT('id', id, 'item', item)) sales FROM sale GROUP BY user) s ON s.user = u.id


JSON_PRETTY(JSON_ARRAYAGG(JSON_OBJECT('id', u.id, 'name', u.name, 'sales', s.sales)))


[
  {
    "id": 1,
    "name": "User 1",
    "sales": [
      {
        "id": 1,
        "item": "t-shirt"
      },
      {
        "id": 2,
        "item": "jeans"
      }
    ]
  },
  {
    "id": 2,
    "name": "User 2",
    "sales": [
      {
        "id": 3,
        "item": "sweatpants"
      },
      {
        "id": 4,
        "item": "gloves"
      }
    ]
  }
]