尝试在“group by”中获取“order by id desc”以仅获取最后一条评论。当我将“in”与>1个元素一起使用时，它的效果很好，但是有一个元素或没有“in”。我的mariadb版本是10.0.36-mariadb-0ubuntu0.16.04.1 ubuntu16.04。
下面是示例：56-真ID，55假ID

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid`='6576') GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55811 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
     WHERE (`commentsapi_comment`.`orderid` IN ('6576')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55811 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576','6576')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 56218 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576','-1')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 56218 |
+-------+
1 row in set (0.01 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55811 |
+-------+
1 row in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6576','6577')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 56199 |
| 56218 |
+-------+
2 rows in set (0.00 sec)

MariaDB [shop]> SELECT `commentsapi_comment`.`id` FROM `commentsapi_comment`
      WHERE (`commentsapi_comment`.`orderid` IN ('6577')) GROUP BY orderid DESC;
+-------+
| id    |
+-------+
| 55813 |
+-------+
1 row in set (0.01 sec)

谁知道原因呢？