Generate multiples rows of new column based on one value of another column - sql

Question

I have a table like below:
ID
Date
1
2022-01-01
2
2022-03-21
I want to add a new column based on the date and it should look like this
ID
Date
NewCol
1
2022-01-01
2022-02-01
1
2022-01-01
2022-03-01
1
2022-01-01
2022-04-01
1
2022-01-01
2022-05-01
2
2022-03-21
2022-04-21
2
2022-03-21
2022-05-21
Let's say that there is a #EndDate = 2022-05-31 (that's where it should stop)
I'm having a hard time trying to figure out how to do it in SSMS. Would appreciate any insights! Thanks :)

Answer 1

In the following solutions we leverage string_split with combination with replicate to generate new records.
select ID
,Date
,dateadd(month, row_number() over(partition by ID order by (select null)), Date) as NewCol
from (
select *
from t
outer apply string_split(replicate(',',datediff(month, Date, '2022-05-31')-1),',')
) t
ID
Date
NewCol
1
2022-01-01
2022-02-01
1
2022-01-01
2022-03-01
1
2022-01-01
2022-04-01
1
2022-01-01
2022-05-01
2
2022-03-21
2022-04-21
2
2022-03-21
2022-05-21
Fiddle
For SQL in Azure and SQL Server 2022 we have a cleaner solution based on [ordinal][4].
"The enable_ordinal argument and ordinal output column are currently
supported in Azure SQL Database, Azure SQL Managed Instance, and Azure
Synapse Analytics (serverless SQL pool only). Beginning with SQL
Server 2022 (16.x) Preview, the argument and output column are
available in SQL Server."
select ID
,Date
,dateadd(month, ordinal, Date) as NewCol
from (
select *
from t
outer apply string_split(replicate(',',datediff(month, Date, '2022-05-31')-1),',',1)
) t

Answer 2

with cal (id, dt) as
(
select id, date as dt from t
union all select id, dateadd(month, 1, dt) from cal where month(dt) < month('2022-05-31')
)
select t.id
,t.date
,cal.dt as new_col
from cal join t on t.id = cal.id and t.date != cal.dt
order by id, new_col
id
date
new_col
1
2022-01-01
2022-02-01
1
2022-01-01
2022-03-01
1
2022-01-01
2022-04-01
1
2022-01-01
2022-05-01
2
2022-03-21
2022-04-21
2
2022-03-21
2022-05-21
Fiddle

Answer 3

There are many ways to "explode" a row into a set, the simplest in my opinion is a recursive CTE:
DECLARE #endpoint date = '20220531';
DECLARE #prev date = DATEADD(MONTH, -1, #endpoint);
WITH x AS
(
SELECT ID, date, NewCol = DATEADD(MONTH, 1, date) FROM #d
UNION ALL
SELECT ID, date, DATEADD(MONTH, 1, NewCol) FROM x
WHERE NewCol < #prev
)
SELECT * FROM x
ORDER BY ID, NewCol;
Working example in this fiddle.
Keep in mind that if you could have > 100 months you'll need to add OPTION (MAXRECURSION) (or just consider using a different solution at scale).