How to join first two child records to parent? - sql-server

Question

I am trying to select the top 25 parent records and join to it the first two child records ordered by date. The parent record can have 0 to n children.
The end result would be something like:
P1, C1, C2
P2, C1, C2
...
P25, C1, C2
I have found an example using max date, but I am having trouble getting a specific row number
select top 25 *
from parentTable p
left join childTable c
on p.Key = c.Key
and c.dateColumn = (
select Max(c.dateColumn)
from c
where p.Key = c.Key
)

Answer 1

You should be able to get what you are looking for using ROW_NUMBER():
WITH c AS
(
SELECT ROW_NUMBER() OVER (PARTITION BY [key] ORDER BY [date] asc) as rn
,[key]
,[date]
from child
)
SELECT top 25 p.[key], c1.[key], c1.[date], c2.[key], c2.[date]
FROM parent p
LEFT JOIN c c1
ON p.[key] = c1.[key]
AND c1.rn = 1
LEFT JOIN c c2
ON p.[key] = c2.[key]
AND c2.rn = 2
Check SQLFiddle for test data/results

Answer 2

You can use CTE.
;with cte as (
select top (25) *
from parentTable
)
select *
from cte p
left join childTable c
on p.[Key] = c.[Key];

Answer 3

First solution (because data set seems to be small) one solution is to use OUTER APPLY:
select top (25) ... columns ...
from parentTable p
outer apply (
select top (2) ... columns ...
from childTable c
where p.[Key] = c.[Key]
order by c.dateColumn desc -- asc ?
) a
-- Most of the times, when top filter is used order by clause should be also used
order by p.dateColumn desc -- asc ?
-- order by p.idColumn desc -- asc ?
Second solution (could be less eficient):
select top (25) ... columns ...
from parentTable p
left join (
select top (2) ... columns ..., ROW_NUMBER() over(partition by c.[Key] order by c.dateColumn desc) as rn -- asc ?
from childTable c
) a on p.[Key] = a.[Key] and a.rn < 3
-- Most of the times, when top filter is used order by clause should be also used
order by p.dateColumn desc -- asc ?
-- order by p.idColumn desc -- asc ?
Note: at least for the first solution one of following indices could help from the point of view of performance:
create index ix_name on dbo.childTable ([Key], [dateColumn])
--or
create index ix_name on dbo.childTable ([Key], [dateColumn])
include (... columns from select top(2) clause ...)