仅当当前表的列与另一个表的列匹配时，才从当前表中选择行

调用此表：customers

+----+-------------+
| id |    name     |
+----+-------------+
|  1 | roger       |
|  2 | steve       |
|  3 | harold      |
|  4 | tom         |
+----+-------------+

这是 table ：orders

+----------+-------------+---------+
| order_id | customer_id |  total  |
+----------+-------------+---------+
|    101   |      1      |    25   |
|    102   |      1      |    18   |
|    103   |      3      |   149   |
|    104   |      4      |   299   |
+----------+-------------+---------+

仅当表中有一行时，如何仅选择表中的行（通过确保匹配（并忽略重复项）？customersorderscustomers.idorders.customer_id