分组查询
正常情况
mysql> SELECT * FROM students ;
+-----+-------------+-----+--------+------------+---------+
| SID | Name | Age | Gender | Tutor | ClassID |
+-----+-------------+-----+--------+------------+---------+
| 1 | Guo Jing | 27 | M | Song Jing | 2 |
| 2 | Yang Guo | 28 | M | Hu Sanning | 3 |
| 3 | Guo Polu | 21 | M | Jia Baoyu | 3 |
| 4 | Xue Baochai | 19 | F | Rong Momo | 1 |
| 5 | Xia Yuhe | 37 | F | Shi Qian | 2 |
| 6 | Wu Yong | 51 | M | Lin Daiyu | 1 |
| 7 | tom | 11 | M | jerry | 1 |
| 9 | tomy | 13 | M | NULL | 4 |
+-----+-------------+-----+--------+------------+---------+
8 rows in set (0.00 sec)
mysql> SELECT * FROM students GROUP BY Gender;
+-----+-------------+-----+--------+-----------+---------+
| SID | Name | Age | Gender | Tutor | ClassID |
+-----+-------------+-----+--------+-----------+---------+
| 4 | Xue Baochai | 19 | F | Rong Momo | 1 |
| 1 | Guo Jing | 27 | M | Song Jing | 2 |
+-----+-------------+-----+--------+-----------+---------+
2 rows in set (0.06 sec)
但是
mysql> SELECT SUM(Age) FROM students GROUP BY Gender HAVING Age > 25;
ERROR 1054 (42S22): Unknown column 'Age' in 'having clause'
居然报错了。。。
