为什么age<20查不出结果呢，明明有3个age小于20的
mysql> SELECT*FROM users3;
+----+----------+----------------------------------+-----+------+
| id | username | password | age | sex |
+----+----------+----------------------------------+-----+------+
| 1 | Tom | 123 | 29 | 0 |
| 2 | John | 456 | 38 | 0 |
| 3 | Tom | 123 | 27 | 0 |
| 4 | Tom | 123 | 27 | 0 |
| 5 | Tom | 123 | 10 | 0 |
| 7 | Rose | 202cb962ac59075b964b07152d234b70 | 8 | 0 |
| 8 | Ben | 456 | 17 | 0 |
| 9 | 111 | 222 | 33 | NULL |
+----+----------+----------------------------------+-----+------+
8 rows in set (0.00 sec)
mysql> SELECT sex,age FROM users3 GROUP BY sex HAVING age<20;
Empty set (0.00 sec)
mysql>
