Join on TOP 1 from subquery while referencing outer tables - sql

Question

I am starting with this query, which works fine:
SELECT
C.ContactSys
, ... a bunch of other rows...
FROM Users U
INNER JOIN Contacts C ON U.ContactSys = C.ContactSys
LEFT JOIN UserWatchList UW ON U.UserSys = UW.UserSys
LEFT JOIN Accounts A ON C.AccountSys = A.AccountSys
WHERE
C.OrganizationSys = 1012
AND U.UserTypeSys = 2
AND C.FirstName = 'steve'
Now, I've been given this requirement:
For every visitor returned by the Visitor Search, take ContactSys, get the most recent entry in the GuestLog table for that contact, then return the columns ABC and XYZ from the GuestLog table.
I'm having trouble with that. I need something like this (I think)...
SELECT
C.ContactSys
, GL.ABC
, GL.XYZ
, ... a bunch of other rows...
FROM Users U
INNER JOIN Contacts C ON U.ContactSys = C.ContactSys
LEFT JOIN UserWatchList UW ON U.UserSys = UW.UserSys
LEFT JOIN Accounts A ON C.AccountSys = A.AccountSys
LEFT JOIN (SELECT TOP 1 * FROM GuestLog GU WHERE GU.ContactSys = ????? ORDER BY GuestLogSys DESC) GL ON GL.ContactSys = C.ContactSys
WHERE
C.OrganizationSys = 1012
AND U.UserTypeSys = 2
AND C.FirstName = 'steve'
Only that's not it because that subquery on the JOIN doesn't know anything about the outer tables.
I've been looking at these posts and their answers, but I'm having a hard time translating them to my needs:
SQL: Turn a subquery into a join: How to refer to outside table in nested join where clause?
Reference to outer query in subquery JOIN
Referencing outer query in subquery
Referencing outer query's tables in a subquery

Answer 1

If that is the logic you want, you can use OUTER APPLY:
SELECT C.ContactSys, GL.ABC, GL.XYZ,
... a bunch of other columns ...
FROM Users U JOIN
Contacts C
ON U.ContactSys = C.ContactSys LEFT JOIN
UserWatchList UW
ON U.UserSys = UW.UserSys LEFT JOIN
Accounts A
ON C.AccountSys = A.AccountSys OUTER APPLY
(SELECT TOP 1 gl.*
FROM GuestLog gl
WHERE gl.ContactSys = C.ContactSys
ORDER BY gl.GuestLogSys DESC
) GL
WHERE C.OrganizationSys = 1012 AND
U.UserTypeSys = 2 AND
C.FirstName = 'steve'