T-SQL Update From Order By - sql-server

Question

I am trying to write a Stored Procedure that will give me Audit information between a date range. The Audit table stores the Audit Date, Column Name and Old Value. I want to display the Old Value and the New Value in the result set. I need to get the new value from the next most recent Audit entry or from the entity itself. The stored procedure is a multi step approach to get the result set I need.
Create a #results temp table with the Audit records from within the date range.
Create a #currentValues temp table with the current values from the entity.
Update the #results table to store the new value
Here is the structure of the Audit Table:
AuditId uniqueidentifier NEWID()
AuditDate datetime GETDATE()
UserId uniqueidentifier
EntityId uniqueidentifier
ColumnName nvarchar(100)
OldValue nvarchar(MAX)
Here is the sql:
CREATE PROC GetAuditSummary
#StartDate datetime = NULL,
#EndDate datetime = NULL
AS
DECLARE #Results table(
AuditId uniqueidentifier,
AuditDate datetime,
UserId uniqueidentifier,
EndityId uniqueidentifier,
ColumnName nvarchar(100),
OldValue nvarchar(MAX),
NewValue nvarchar(MAX)
INSERT INTO #Results(AuditId, AuditDate, UserId, EntityId, ColumnName, OldValue)
SELECT AuditId, AuditDate, UserId, EntityId, ColumnName, OldValue
FROM Audit
WHERE (AuditDate >= #StartDate) AND (AuditDate < #EndDate)
DECLARE #CurrentValues table(
EntityId uniqueidentifier,
ColumnName nvarchar(100),
Value nvarchar(MAX)
)
--Lengthy Code to fill #CurrentValues temp table. Assume #CurrentValues is populated
UPDATE #Results
SET NewValue = n.Value
FROM #Results r INNER JOIN
(SELECT AUditId, AuditDate, EntityId, ColumnName, OldValue AS Value
FROM Audit
UNION ALL
SELECT NULL, GETDATE(), EntityId, ColumnName, Value
FROM #CurrentValues
ORDER BY AuditDate DESC
) n ON n.EntityId = r.EntityId AND
n.ColumnNmae = r.ColumName NAD
n.AuditDate > r.AuditDate
SELECT * FROM #Results ORDER BY AuditDate DESC
Now, and correct me if I'm wrong, when the update statement executes, the NewValue should be set to the last matching row in the joined result set and since I have the subquery ordered by AuditDate, the AuditDate closest to the current record from #Results should be the value that's set to NewValue. I've tried this, but I get an error telling me I can't use an Order By in a subquery. Is there another way to do this? I'm open to any suggestions, but I need to take performance into consideration as there is a chance of having thousands of rows in the result.
--EDIT
Here is one way to get it working, but I'm not sure it's the best on performance.
UPDATE #Results
SET NewValue = COALESCE(
(SELECT TOP 1 a.OldValue
FROM Audit a
WHERE (a.EntityId = r.EntityId) AND
(a.ColumnName = r.ColumnName) AND
(a.AuditDate > r.AuditDate)
ORDER BY a.AuditDate),
(SELECT TOP 1 c.Value
FROM #CurrentValues c
WHERE (c.EntityId = r.EntityId) AND
(c.ColumnName = r.ColumnName))
FROM #Results r

Answer 1

I would use Row_Number or Rank function to get the row after last matching row.
Following example should work, you may want to change (order by n.AuditDate) to (order by n.AuditDate desc) if you want most recent record after matching date.
UPDATE #Results
SET NewValue = n.Value
FROM #Results r
INNER JOIN
(
select n.EntityId, n.ColumnName, n.Value, Row_Number() over(partition by n.EntityId, n.ColumnName order by n.AuditDate) RowNumber
from
#Results ir
inner join (
SELECT AuditDate, EntityId, ColumnName, OldValue AS Value
FROM Audit
UNION ALL
SELECT GETDATE(), EntityId, ColumnName, Value
FROM #CurrentValues
) inn on inn.EntityId = ir.EntityId AND
inn.ColumnNmae = ir.ColumName NAD
inn.AuditDate > ir.AuditDate
) n ON n.EntityId = r.EntityId AND
n.ColumnNmae = r.ColumName AND
n.RowNumber = 1