Store the result of a Dynamic Query in a variable - sql

Question

I know this question has been asked, and I already found some solutions in internet.. but I still can not make it work properly.
So.. I have to make a SELECT query and store the result in a variable (I DONT want a table variable).
My problem is that the name of the table is also a variable. The table name changes accordingly to a WHILE, here is my code:
DECLARE #numRecord INT;
DECLARE #maxMacNumber INT;
SET #maxMacNumber = 500;
DECLARE #mac INT;
SET #mac = 0;
DECLARE #res FLOAT;
DECLARE #ap INT;
SET #ap = 0;
DECLARE #apString VARCHAR(2);
DECLARE #numRecordString VARCHAR(20);
DECLARE #tablename VARCHAR(500);
DECLARE #sql NVARCHAR(500);
DECLARE #varDefinition NVARCHAR(200);
WHILE #mac <= #maxMacNumber
BEGIN
SET #numRecord = 6 + #mac * 390;
SET #ap = 0;
WHILE #ap < 2
BEGIN
SELECT #apString = CONVERT(VARCHAR,#ap);
SELECT #numRecordString = CONVERT(VARCHAR, #numRecord);
SELECT #rssiString = CONVERT(VARCHAR, #rssi);
SET #tablename = 'APDB.dbo.AP' + #apString;
SET #sql = 'SELECT RSSI FROM ' + #tablename + ' WHERE ID=' + #numRecordString;
SET #varDefinition = N'#res FLOAT OUTPUT';
EXEC sp_executesql #sql, #varDefinition, #res = #res OUTPUT;
PRINT #res;
-- HERE I WILL DO SOMETHING WITH #res
END;
END;
The problem is that it doesn't print anything when I do PRINT #res...

Answer 1

This is the relevant SQL code:
SET #sql = 'SELECT RSSI FROM ' + #tablename + ' WHERE ID=' + #numRecordString;
SET #varDefinition = N'#res FLOAT OUTPUT';
EXEC sp_executesql #sql, #varDefinition, #res = #res OUTPUT;
PRINT #res;
You are never setting #res in the SQL. Try this:
SET #sql = 'SELECT #res = RSSI FROM ' + #tablename + ' WHERE ID=' + #numRecordString;
SET #varDefinition = N'#res FLOAT OUTPUT';
EXEC sp_executesql #sql, #varDefinition, #res = #res OUTPUT;
PRINT #res;