MySQL 5.6

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create table `grey_csgo_gifts_list` (
  `id` int(11) NOT NULL,
  `username` varchar(32) NOT NULL
);
insert into `grey_csgo_gifts_list` values
  (0, 'User 1'), (0, 'User 2');
select * from `grey_csgo_gifts_list`;


id
username


0
User 1

0
User 2


set @i = 0;

update `grey_csgo_gifts_list`
  set id = @i := @i + 1;
select * from `grey_csgo_gifts_list`;


id
username


1
User 1

2
User 2


alter table `grey_csgo_gifts_list` modify `id` int(11) auto_increment primary key;
insert into `grey_csgo_gifts_list` (`username`) values
  ('User 3'), ('User 4'), ('User 5');
select * from `grey_csgo_gifts_list`;


id
username


1
User 1

2
User 2

3
User 3

4
User 4

5
User 5