SQL - slow While loop - sql

Question

I have the following T-SQL code that is either adding or updating 1 record at a time to a temp table. Does anyone have any suggestions to speed this process up?
DECLARE #Total AS INT
SELECT #Total = count(AgentsID) from #TempAgentsConcat
DECLARE #counter AS INT
SET #counter = 1
DECLARE #CurrentVal AS NVARCHAR(1024)
DECLARE #RowCount AS INT
DECLARE #OBJ_ID AS INT
while (#counter <= #Total)
begin
SELECT #OBJ_ID = Id FROM #TempAgentsConcat WHERE AgentsId = #counter
SELECT #CurrentVal = SVRMachine FROM #TempAgentsConcat WHERE ID = #OBJ_ID
IF EXISTS (SELECT * FROM #TempEndpoints WHERE ID = #OBJ_ID)
BEGIN
UPDATE #TempEndpoints SET SVRMachine = #CurrentVal WHERE ID = #OBJ_ID
END
ELSE
BEGIN
INSERT INTO #TempEndpoints (SVRMachine, IPPort, ID)
VALUES (#CurrentVal, NULL, #OBJ_ID)
END
--END
SET #counter = #counter + 1
end

Answer 1

It looks like, you are trying to merge one table into other. First lets talk of couple of issues in your query-
1. Avoid using loops unless it's extremely necessary.
2. You are assigning two different variables by reading same row in 2 queries.
You can do this in single query like
SELECT #OBJ_ID = Id,#CurrentVal = SVRMachine FROM #TempAgentsConcat WHERE AgentsId = #counter
instead of 2 queries
SELECT #OBJ_ID = Id FROM #TempAgentsConcat WHERE AgentsId = #counter
SELECT #CurrentVal = SVRMachine FROM #TempAgentsConcat WHERE ID = #OBJ_ID
Let's rewrite the query without using loops. The answer by #Cetin is one of the solutions. Your requirement looks classic example of merging tables, so you may use SQL MERGE (SQL server 2008 and above). You can read more about MERGE, here, checkout the example 'C'.
Using MERGE, your query will look like below.
MERGE INTO #TempEndpoints AS Target
USING (SELECT SVRMachine , Id from #TempAgentsConcat)
AS Source (SVRMachine, ID)
ON Target.ID = Source.ID
WHEN MATCHED THEN
UPDATE SET SVRMachine = Source.SVRMachine
WHEN NOT MATCHED BY TARGET THEN
INSERT (ID, IPPort, SVRMachine) VALUES (Id, NULL,SVRMachine)

Answer 2

Why would you use a lot of variables and loop. SQL server (any SQL series database as well) works best with sets, rather than loops:
UPDATE #TempEndpoints
SET SVRMachine = ac.SVRMachine
FROM #TempAgentsConcat ac
WHERE #TempEndpoints.Id = ac.ID;
INSERT INTO #TempEndpoints
(
SVRMachine,
ID
)
SELECT SVRMachine,
ID
FROM #TempAgentsConcat ac
WHERE NOT EXISTS
(
SELECT * FROM #TempEndpoints ep WHERE ep.ID = ac.ID
);