SQL Server 2017

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

create table docs
( doc_id int not null primary key,
  dt datetime not null
) ;
insert into docs (doc_id, dt)
values
(1, '2000-01-01 00:00:00'),
(2, '2001-05-12 12:30:00'),
(3, '2007-07-07 16:45:00'),
(4, '2018-01-01 12:00:00'),
(5, '2018-05-15 15:50:00') ;
5 rows affected

select * from docs;


doc_id
dt


1
01/01/2000 00:00:00

2
12/05/2001 12:30:00

3
07/07/2007 16:45:00

4
01/01/2018 12:00:00

5
15/05/2018 15:50:00


alter table docs
add year_month as year(dt) * 100 + month(dt) ;
select * from docs;


doc_id
dt
year_month


1
01/01/2000 00:00:00
200001

2
12/05/2001 12:30:00
200105

3
07/07/2007 16:45:00
200707

4
01/01/2018 12:00:00
201801

5
15/05/2018 15:50:00
201805

doc_id	dt
1	01/01/2000 00:00:00
2	12/05/2001 12:30:00
3	07/07/2007 16:45:00
4	01/01/2018 12:00:00
5	15/05/2018 15:50:00

doc_id	dt	year_month
1	01/01/2000 00:00:00	200001
2	12/05/2001 12:30:00	200105
3	07/07/2007 16:45:00	200707
4	01/01/2018 12:00:00	201801
5	15/05/2018 15:50:00	201805