group by：把数据分为多个逻辑组，并对每个逻辑进行操作。
having：用来过滤分组。
mysql> select * from friut;
+------+------+--------+---------+
| f_id | s_id | f_name | f_price |
+------+------+--------+---------+
| a | 102 | apple | 5 |
| b | 103 | orange | 9 |
| c | 104 | melon | 2 |
| d | 105 | banana | 8 |
| e | 102 | grape | 2 |
| f | 104 | mango | 8 |
+------+------+--------+---------+
6 rows in set (0.00 sec)
mysql> select * from friut group by s_id;
ERROR 1055 (42000): Expression #1 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'mapan.friut.f_id' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
我直接查询并通过s_id进行分组直接报错。这个查询语句分组了，但是还是直接显示所有，有些字段分组不能显示，所以报错。
mysql> select s_id from friut group by s_id;
+------+
| s_id |
+------+
| 102 |
| 103 |
| 104 |
| 105 |
+------+
4 rows in set (0.00 sec)
这次执行成功了，我只选取分组之后s_id结果。
mysql> select s_id,count(s_id) from friut group by s_id;
+------+-------------+
| s_id | count(s_id) |
+------+-------------+
| 102 | 2 |
| 103 | 1 |
| 104 | 2 |
| 105 | 1 |
+------+-------------+
4 rows in set (0.00 sec)
mysql>
这次不仅分组了，还计算出了每组对应s_id的数量。group by通常与MAX(),MIN(),COUNT(),SUM(),AVG()这些函数一起使用。
mysql> select s_id,count(s_id) as total from friut group by s_id having total>1 ;
+------+-------+
| s_id | total |
+------+-------+
| 102 | 2 |
| 104 | 2 |
+------+-------+
2 rows in set (0.00 sec)
mysql>
加上having过滤条件。很简单，但也很容易忘记。