Transform rows to comma delimited string by groups of n records

Question

Based on this sample table

ID	NAME	ZIP	NTID
1	Juan	123	H1
1	Juan	123	H2
2	John	456	H3
2	John	456	H4
2	John	456	H5

I want to show ntid as a comma separated value but in groups of maximum 2 items

Expected result

ID	NAME	ZIP	NTID
1	Juan	123	H1, H2
2	John	456	H3, H4
2	John	456	H5

Using SQL Server, is there a way to achieve this?

By using the STRING_AGG function it concatenates all in a single row, but I need to group as groups of maximum of 2 members.

SELECT ID, NAME, ZIP, STRING_AGG(NTID,',')
FROM MyTable
GROUP BY ID, NAME, ZIP

Answer 1

So you can use a modified row number (divided by 2) to group your data into blocks of 2 rows, and then you can use string_agg() .

You do need a way to order the rows though (if you want consistent results), I have assumed NTID will work, but you may have a better column to order by.

declare @Test table (ID int, [NAME] varchar(32), ZIP varchar(12), NTID varchar(2));

insert into @Test (ID, [NAME], ZIP, NTID)
values
(1, 'Juan', '123', 'H1'),
(1, 'Juan', '123', 'H2'),
(2, 'John', '456', 'H3'),
(2, 'John', '456', 'H4'),
(2, 'John', '456', 'H5');

with cte as (
    select *
      , (row_number() over (partition by ID order by NTID) - 1) / 2 rn
    from @Test
)
select ID, [NAME], string_agg(NTID,',')
from cte
group by ID, [NAME], rn;

db<>fiddle here

Note: If you supply the DDL+DML as part of your question you make it much easier to answer.