group by：把資料分為多個邏輯組，並對每個邏輯進行操作。

          having：用來過濾分組。

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
我直接查詢並通過s_id進行分組直接報錯。這個查詢語句分組了，但是還是直接顯示所有，有些欄位分組不能顯示，所以報錯。

mysql> select s_id from friut group by s_id;
+------+
| s_id |
+------+
|  102 |
|  103 |
|  104 |
|  105 |
+------+
4 rows in set (0.00 sec)
這次執行成功了，我只選取分組之後s_id結果。

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> 
這次不僅分組了，還計算出了每組對應s_id的數量。group by通常與MAX(),MIN(),COUNT(),SUM(),AVG()這些函式一起使用。

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過濾條件。很簡單，但也很容易忘記。