mysql avg()函数，获取字段的平均值

mysql> select * from table1;
+----------+------------+-----+---------------------+
| name_new | transactor | pid | order_date          |
+----------+------------+-----+---------------------+
| 1hahha   | 1xiaohong  |    | -- :: |
| 2hahha   | 2xiaohong  |    | -- :: |
| 3hahha   | 3xiaohong  |    | -- :: |
| 4hahha   | 4xiaohong  |    | -- :: |
| 3hahha   | bob        |    | -- :: |
| 3hahha   | lee        |    | -- :: |
+----------+------------+-----+---------------------+
 rows in set (0.00 sec)

mysql> select avg(pid) from table1;
+----------+
| avg(pid) |
+----------+
|   3.5000 |
+----------+
 row in set (0.00 sec)

mysql> select (/);
+--------+
| (/) |
+--------+
| 3.5000 |
+--------+
 row in set (0.00 sec)