Does MERGE updates when not needed? In SQL Server 2012,

Does a merge statement like this :

WHEN MATCHED THEN
update SET A.len = B.len


updates a row even if A.len equals to B.len ?

Or does the SQL SERVER knows to ignore this situations ?

If not, would it be better to add the matching condition the following :

WHEN MATCHED AND A.len &lt;&gt; B.len THEN
update SET A.len = B.len


Is there any difference in this on other Sql servers ?

Thanks,
The short answer is yes. You can read more about the Merge command syntax at the Microsoft MSDN site.