sql server 有则更新无则插入

1   string sql = " IF EXISTS(SELECT * FROM ****** WHERE ****** )  ";
2             sql += " BEGIN ";
3             sql += "  UPDATE  *****  SET  *****  WHERE **** ";
4             sql += " END ELSE BEGIN  ";
5             sql += "INSERT INTO **** (*，*，*，*)  values (*，*，*，*)      ";
6             sql += " END";