And if you must use a loop then how about this?

declare @Str1 varchar(10) = 'ABBA';

declare @IsPalindrome bit = 1;

declare @LoopCnt int = 0;

while @LoopCnt < len(@Str1) and @IsPalindrome = 1

begin

select @IsPalindrome = case when substring(@Str1,1 + @LoopCnt,1) = substring(@Str1,len(@Str1) - @LoopCnt,1) then 1 else 0 end;

set @LoopCnt = @LoopCnt + 1;

end

if @IsPalindrome = 1

print @Str1 + ' is a palindrome'

else

print @Str1 + ' is not a palindrome'

set @LoopCnt = 0;

set @IsPalindrome = 1;

set @Str1 = 'ABC';

while @LoopCnt < len(@Str1) and @IsPalindrome = 1

begin

select @IsPalindrome = case when substring(@Str1,1 + @LoopCnt,1) = substring(@Str1,len(@Str1) - @LoopCnt,1) then 1 else 0 end;

set @LoopCnt = @LoopCnt + 1;

end

if @IsPalindrome = 1

print @Str1 + ' is a palindrome'

else

print @Str1 + ' is not a palindrome'

set @LoopCnt = 0;

set @IsPalindrome = 1;

set @Str1 = 'ABA';

while @LoopCnt < len(@Str1) and @IsPalindrome = 1

begin

select @IsPalindrome = case when substring(@Str1,1 + @LoopCnt,1) = substring(@Str1,len(@Str1) - @LoopCnt,1) then 1 else 0 end;

set @LoopCnt = @LoopCnt + 1;

end

if @IsPalindrome = 1

print @Str1 + ' is a palindrome'

else

print @Str1 + ' is not a palindrome'

You will also notice that I short circuit my loop to avoid unnecessary comparisons.