How to execute an UPDATE only if one row would be affected?

Question

I have a table in SQL Server that has a PK (ID) and another (logical) primary key made by a couple of other columns (although there is no UNIQUE constraint on that)

Answer 1

UPDATE Person p1
SET p1.AGE = 43
WHERE NAME = 'XX' AND SURNAME = 'YY'
AND NOT EXISTS (
  SELECT NULL FROM Person p2
  WHERE p1.NAME = p2.NAME
  AND p1.SURNAME = p2.SURNAME
  AND p1.ID <> p2.ID)