Group By:

    Group by子句用于按指定的一些列汇总显示数据行和列。它可用于完成聚合函数，例如count()。

    以下的查询示例通过在select语句中使用Group By子句将列出学生的名字，并对重复的名字进行计数。

mysql> select name, count(name) from student group by name;
+-------+-------------+
| name  | count(name) |
+-------+-------------+
| anne  |           2 |
| david |           1 |
| jack  |           1 |
| mille |           1 |
| steve |           2 |
+-------+-------------+
5 rows in set (0.03 sec)

mysql> select name,sum(marks),count(*) from students group by name;
+----------+------------+----------+
| name     | sum(marks) | count(*) |
+----------+------------+----------+
| anne     |        175 |        2 |
| maichael |         82 |        1 |
| mike     |        182 |        2 |
| rock     |        100 |        1 |
| steve    |        175 |        2 |
+----------+------------+----------+
5 rows in set (0.00 sec)