SQL Select maxDate in SP - sql

Question

USE [MyDatabase]
GO
Object: StoredProcedure [dbo].[SP_MyProcedure]
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE PROCEDURE [dbo].[SP_MyProcedure]
-- Add the parameters for the stored procedure here
#StartDate NVARCHAR(19),
#EndDate NVARCHAR(19)
AS
BEGIN
SET NOCOUNT ON;
Insert statements for procedure here
DECLARE #FirstQuery nvarchar(1500);
DECLARE #SecondQuery nvarchar(1500);
DECLARE #TSQL nvarchar(4000);
SET #FirstQuery =
'
SELECT * FROM OPENQUERY(LinkedServer,
''
SELECT * FROM Server.Table
WHERE Name IN
(SELECT Tagname COLLATE DATABASE_DEFAULT FROM LocalServer.MyServer.dbo."NameList"
WHERE LOCATION = ''''X'''' AND SOURCE = ''''Y'''') AND TIMESTAMP >= ''''' + #StartDate + ''''' AND TIMESTAMP < ''''' + #EndDate + ''''''')';
My question is: How could I include max(date) in the code, so the #startDate would be compared to the latest available date for a column in the DB instead of todays date.
The code you see works fine, however when I choose a later date than the last existing date in the DB the query runs for a while before it returns an error that no columns were found.
Thanks in advance for any suggetions.
HELP.
The following part of the code doesn't work, but I can't figure out why. Does anyone have any suggestions? Thank you
-- Insert statements for procedure here
DECLARE #FirstQuery nvarchar(1500);
DECLARE #SecondQuery nvarchar(1500);
DECLARE #TSQL nvarchar(4000);
DECLARE #MaxTimeStamp nvarchar(19);
SET#MaxTimeStamp =
'SELECT MAX(TimeStamp) From OPENQUERY(LinkedServer)'
IF #StartDate <= #MaxTimeStamp
BEGIN
SET #FirstQuery =
'
SELECT * FROM OPENQUERY(LinkedServer,
''
SELECT * FROM Server.Table
WHERE Name IN
(SELECT Tagname COLLATE DATABASE_DEFAULT FROM LocalServer.MyServer.dbo."NameList"
WHERE LOCATION = ''''X'''' AND SOURCE = ''''Y'''') AND TIMESTAMP >= ''''' + #StartDate + ''''' AND TIMESTAMP < ''''' + #EndDate + ''''''')';
END

Answer 1

Well, you could replace it with a subquery:
(select max(timestamp) from NameList)
If this is a stored procedure, you might want to put this in a variable, something like:
declare #MaxTimestamp datetime;
select #MaxTimestamp = max(TimeStamp) from NameList
-- in your query, something like
coalesce(#StartTime, maxTimeStamp)
If performance is an issue, try adding an index on NameList(Location, Source, Timestamp).

Answer 2

Try this...
DECLARE #maxDate datetime;
SELECT #maxDate = MAX(timestamp) FROM OPENQUERY(...);
IF #StartDate <= #maxDate
BEGIN
--Your original query here
END