1、当没有记录的时候，使用聚集函数，会导致出现一条记录，记录的取值都是NULL，如下：
mysql> select name from student where name='David';
Empty set
mysql> select name,avg(age) from student where name='David';
+------+----------+
| name | avg(age) |
+------+----------+
| NULL | NULL |
+------+----------+
1 row in set
2、对于取值为NULL的记录，不参与聚集函数的计算。注意：取值为NULL，也就是没有值，与取值为0，或者''不是一个概念。如下：
mysql> select * from student;
+----+--------+----------+------+
| ID | SCHOOL | NAME | AGE |
+----+--------+----------+------+
| 1 | NUM_1 | Andy | 35 |
| 2 | NUM_1 | Andy | 31 |
| 3 | NUM_2 | Bill | 28 |
| 4 | NUM_2 | Caroline | 30 |
| 5 | NUM_3 | Bill | 26 |
| 9 | NUM_2 | Andy | NULL |
+----+--------+----------+------+
6 rows in set
mysql> select name,avg(age) from student where name='Andy';
+------+----------+
| name | avg(age) |
+------+----------+
| Andy | 33.0000 |
+------+----------+
1 row in set
mysql> update student set age=0 where id =9;
Query OK, 1 row affected
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select name,avg(age) from student where name='Andy';
+------+----------+
| name | avg(age) |
+------+----------+
| Andy | 22.0000 |
+------+----------+
1 row in set
