Ok, IMHO, the problem here is the use of WHILE EXISTS. Using SET has more serious problems. You should always use SELECT whenever you can, instead of SET.

If you're using SET to assign a value to a variable, and the SQL fails, the variable retains the previous value. NOT good! If you use SELECT, at least NULL will be assigned to the variable. FAR better.

--Quick and dirty example: DECLARE @t varChar(25); SET @t = 'Test 1';

--Some work here...

SET @t = 'Test 2' + ' ' + SELECT TOP 1 SomeSQLThatFails FROM someServer.dbo.SomeTable;

SELECT @t; -- THIS RETURNS 'Test 1' IF the SQL in your SET statement above fails.

SELECT @t = 'Test 2' + ' ' + SELECT TOP 1 SomeSQLThatFails FROM someServer.dbo.SomeTable;

SELECT @t; -- THIS returns NULL. Better NULL than some incorrect value.

--The SQL Whisperer

I would use @@ROWCOUNT, and include an order BY for the select top 1 :

TRUNCATE TABLE ToTab;

DECLARE @Var INT;
DECLARE @Startdate DATETIME;

SET @Startdate = Getdate();

SELECT TOP 1 @Var = Id FROM FromTab ORDER BY Id;
WHILE @@ROWCOUNT = 1
BEGIN
   PRINT CAST(@Var AS VARCHAR) + ' : ' + CONVERT(VARCHAR,DATEDIFF(SECOND,@Startdate,GETDATE())) + ' Seconds';

   INSERT INTO ToTab (Id,Name) SELECT Id,Name FROM FromTab WHERE Id = @Var;

   SELECT TOP 1 @Var = Id FROM FromTab WHERE Id > @Var ORDER BY Id;
END;
GO

There are two very basic coding errors in your example: 1. The loop has a WHILE condition that will never be satisfied, so loops infinitely. 2. The TOP operator is used without an ORDER BY clause, so the start condition is not defined.

Your WHILE loop has the end condition

WHILE EXISTS (SELECT
           1
         FROM   FromTab
         WHERE  Id <= @Var

The variable @Var starts by being initialised with a Id value from the FromTab table. Each iteration of the loop sets @Var to a higher value, so there will always exist an Id less than @Var

This is just a very basic logic error. The end condition should be

EXISTS(SELECT
           1
         FROM   FromTab
         WHERE  Id > @Var)

which does complete after (usually) four iterations.