CREATE TABLE a (
  id int(11) NOT NULL AUTO INCREMENT,
  b varchar(10) NOT NULL,
  d date NULL,
  PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=latin1; //

CREATE PROCEDURE CheckNoDuplicates (new_id int(11), new_b varchar(10), new_d date)
BEGIN
  DECLARE existingId INT(11);

  SELECT id INTO existingId
  FROM a
  WHERE new_b = b
      AND d IS NOT NULL AND new_d IS NOT NULL AND YEAR(d) = YEAR(new_d)
        OR d IS NULL AND new_d IS NULL
      )
      AND id <> new_id;

  IF existingId IS NOT NULL THEN
    SIGNAL SQLSTATE '23000';
  END IF;
END //

CREATE TRIGGER NoDuplicateOnInsert BEFORE INSERT ON a
FOR EACH ROW
  BEGIN
    CALL CheckNoDuplicates(NEW.id, NEW.b, NEW.d);
  END //

CREATE TRIGGER NoDuplicateOnInsert BEFORE UPDATE ON a
FOR EACH ROW
  BEGIN
    CALL CheckNoDuplicates(NEW.id, NEW.b, NEW.d);
  END //