create table player (
  id serial primary key,
  text varchar(20)
);

create table item (
  id BIGINT UNSIGNED primary key,
  text varchar(20)
);

create table player_item (
  id serial primary key,
  player BIGINT UNSIGNED NOT NULL,
  item BIGINT UNSIGNED NOT NULL,
  text varchar(20),
  FOREIGN KEY (player) REFERENCES Player(id),
  FOREIGN KEY (item) REFERENCES Item(id)
);

update mysql.user set host='localhost' where user = 'user1';
update mysql.user set host='localhost' where user = 'user2';

grant all privileges on *.* to 'user1'@'localhost';
grant all privileges on *.* to 'user2'@'localhost';

insert into player (id, text) values (100,'player1');
insert into item (id, text) values (2000, 'item1');

insert into player_item (player, item) values (100, 2000);
select * from player where id=100 for update;
update player set text='player2' where id=100;

create table player_item (
  id serial primary key,
  player BIGINT UNSIGNED NOT NULL,
  item BIGINT UNSIGNED NOT NULL,
  text varchar(20),
  index idx_item (item),
  index idx_player (player),
  FOREIGN KEY (player) REFERENCES Player(id),
  FOREIGN KEY (item) REFERENCES Item(id)
);

select * from player where id = 100 for update;
insert into player_item (player, item) values (100, 2000);
select * from item where id=2000 for update;

create table player_token (
  id serial primary key,
  token varchar(20) NOT NULL,
  item BIGINT UNSIGNED NOT NULL,
  unique key idx_token (token),
  index idx_item (item),
  FOREIGN KEY (id) REFERENCES Player(id),
  FOREIGN KEY (item) REFERENCES Item(id)
);

insert into item (id, text) values (1000, 'item2');
insert into player (id, text) values (200,'player2');
insert into player_token (id, item, token) values (100, 1000, 'ABCD');
insert into player_token (id, item, token) values (200, 2000, 'ABCD'); 
select * from item where id = 2000 for update;
select * from player where id = 200 for update;