SQL subquery with latest record - sql

Question

I've read just about every question on here that I can find that is referencing getting the latest record from a subquery, but I just can't work out how to make it work in my situation.
I'm creating an SSRS report for use on SQL Server 2008.
In the database is a table of contacts and DBSdata. I want to pull up a list of contacts and the latest record (many of the fields from that row) from the DBSdata table (expiry date furthest in the future)
Contacts
========
PKContactID ContactName
----------- -----------
1 JONES Chris
2 SMITH Mary
3 GREY Jean
DBSdata
=======
Ordinal FKContactID ExpiryDate IssueDate DBSType
------- ----------- ---------- --------- -------
3 1 2021-09-01 2019-09-01 Internal
2 1 2019-08-31 2017-08-31 External
1 1 2017-07-01 2015-07-01 Internal
2 2 2021-04-15 2019-04-15 Internal
1 2 2019-05-05 2017-05-06 External
1 3 2018-01-03 2016-03-02 External
And the result I'd like is:
Latest DBS
==========
PKContactID ContactName ExpiryDate IssueDate DBSType
-------------------------------------------------------------------
3 GREY Jean 2018-01-03 2016-03-02 External
1 JONES Chris 2021-09-01 2019-09-01 Internal
2 SMITH Mary 2021-04-15 2019-04-15 Internal
[The DBSData table doesn't have it's own Primary Key field - that's not something I have control over, unfortunately... And the ordinal increases per contact, so FKContactID+Ordinal is unique....]
This is the code I've kind of got to, but it isn't working. The system I'm uploading the SSRS to doesn't give me any useful error message at all, so I can't be more specific about what isn't working I'm afraid. I get none of the SSRS report displayed, just an error saying the dataset source isn't working.
SELECT
c.PKContactID, c.ContactName, d.ExpiryDate, d.IssueDate, d.DBSType
FROM
Contacts c
LEFT JOIN (
SELECT TOP 1 FKContactID, ExpiryDate, IssueDate, DBSType
FROM DBSData
WHERE FKContactID = c.PKContactID
ORDER BY ExpiryDate DESC
) d ON c.PKContactID = d.FKContactID
ORDER BY
c.ContactName
I suspect it's something to do with that WHERE in the subquery, but if I don't have that, that whole table is using the WHOLE table and returning 1 row, not the top 1 for that contact.

Answer 1

Your method would work using APPLY, instead of JOIN:
SELECT c.PKContactID, c.ContactName,
d.ExpiryDate, d.IssueDate, d.DBSType
FROM Contacts c OUTER APPLY
(SELECT TOP 1 d.*
FROM DBSData d
WHERE d.FKContactID = c.PKContactID
ORDER BY d.ExpiryDate DESC
) d
ORDER BY c.ContactName;
Technically APPLY implements something called a lateral join. This is like a correlated subquery, but it can return multiple rows and multiple columns. Lateral joins are very powerful, and this is a good example for using them.
For performance, you want indexes on DBSData(FKContactID, ExpiryDate DESC) (perhaps including the other columns you want as well) and Contacts(ContactName).
With the right indexes, I would expect this to have performance at least as good as other methods.
An alternative that also typically has good performance is using a correlated subquery for filtering:
SELECT c.PKContactID, c.ContactName,
d.ExpiryDate, d.IssueDate, d.DBSType
FROM Contacts c LEFT JOIN
DBSData d
ON d.FKContactID = c.PKContactID AND
d.ExpiryDate = (SELECT MAX(d2.ExpiryDate)
FROM DBSData d
WHERE d2.FKContactID = d.FKContactID
);
Note that to match the LEFT JOIN, the correlation condition needs to be in the ON clause, not the WHERE clause.
Finally, if you do use window functions, I would recommend a subquery for getting the first row:
SELECT c.PKContactID, c.ContactName,
d.ExpiryDate, d.IssueDate, d.DBSType
FROM Contacts c LEFT JOIN
(SELECT d.*,
ROW_NUMBER() OVER (PARTITION BY d.FKContactID ORDER BY d.PKContactID DESC) as seqnum
FROM DBSData d
) d
ON d.FKContactID = c.PKContactID AND
d.seqnum = 1;
Doing the subquery before the JOIN gives more opportunities for the optimizer to produce a better execution plan.

Answer 2

Here's one option using row_number():
SELECT *
FROM (
SELECT
c.PKContactID, c.ContactName, d.ExpiryDate, d.IssueDate, d.DBSType,
row_number() over (partition by c.PKContactID order by d.ExpiryDate desc) rn
FROM
Contacts c
LEFT JOIN DBSData d ON d.FKContactID = c.PKContactID
) t
WHERE rn = 1
ORDER BY ContactName
Online Demo

Answer 3

This Solution gives result as you expected and performance is so much higher.
select c.PKContactID,c.ContactName,d.ExpiryDate, d.IssueDate, d.DBSType from Contacts c
inner join DBSdata d
on c.PKContactID=d.FKContactID
where d.Ordinal in (select max(d.Ordinal) from DBSdata d where d.FKContactID=c.PKContactID)
order by c.ContactName