distinct关键字，可以数据去重。
mysql> select distinct id from products;
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
+------+
3 rows in set (0.01 sec)
distinct关键字，可以数据去重。
mysql> select distinct id from products;
+------+
| id |
+------+
| 1 |
| 2 |
| 3 |
+------+
3 rows in set (0.01 sec)
