mysql avg()函数，获取字段的平均值

mysql> select * from table1;
+----------+------------+-----+---------------------+
| name_new | transactor | pid | order_date          |
+----------+------------+-----+---------------------+
| 1hahha   | 1xiaohong  |   1 | 2019-08-04 20:45:47 |
| 2hahha   | 2xiaohong  |   2 | 2019-08-04 20:45:57 |
| 3hahha   | 3xiaohong  |   3 | 2019-08-04 20:46:02 |
| 4hahha   | 4xiaohong  |   4 | 2019-08-04 20:46:05 |
| 3hahha   | bob        |   5 | 2019-08-04 20:46:08 |
| 3hahha   | lee        |   6 | 2019-08-04 20:46:12 |
+----------+------------+-----+---------------------+
6 rows in set (0.00 sec)


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


mysql> select (21/6);
+--------+
| (21/6) |
+--------+
| 3.5000 |
+--------+
1 row in set (0.00 sec)