Anonymous

create table nulltest(a int, b int, name text);
insert into nulltest values(1, 1, 'eka');
insert into nulltest values(2, 2, 'toka');
insert into nulltest values(1, null, 'yksi null');
insert into nulltest values(null, 5, 'null viisi');

mysql> select * from nulltest;
+------+------+------------+
| a    | b    | name       |
+------+------+------------+
|    1 |    1 | eka        |
|    2 |    2 | toka       |
|    1 | NULL | yksi null  |
| NULL |    5 | null viisi |
+------+------+------------+
4 rows in set (0.00 sec)

mysql> select sum(a + b), name from nulltest;
+------------+------+
| sum(a + b) | name |
+------------+------+
|          6 | eka  |
+------------+------+
1 row in set (0.01 sec)

mysql> select a + b, name from nulltest;
+-------+------------+
| a + b | name       |
+-------+------------+
|     2 | eka        |
|     4 | toka       |
|  NULL | yksi null  |
|  NULL | null viisi |
+-------+------------+
4 rows in set (0.00 sec)

mysql> select sum(a) from nulltest;
+--------+
| sum(a) |
+--------+
|      4 |
+--------+
1 row in set (0.00 sec)

mysql> select sum(b) from nulltest;
+--------+
| sum(b) |
+--------+
|      8 |
+--------+
1 row in set (0.00 sec)

mysql> select sum(a + b) from nulltest;
+------------+
| sum(a + b) |
+------------+
|          6 |
+------------+
1 row in set (0.00 sec)