如上所述，这个问题只能引起猜测鉴于你所提供的。这就是说，这里有一个简单的方法来调试出现这样的抓头程序：

SELECT visitorIP
  FROM visitors
 WHERE visitDate BETWEEN '2018-08-18 00:00:00' AND '2018-08-19 20:32:03'
    AND visitorIP NOT IN (
        # query 1
        UNION ALL
        # query 2
    )
 ORDER 
    BY visitDate DESC

如果第一个查询中的结果没有显示这两个子查询的结果，这将显示它们是什么，并且从中您应该能够确定逻辑中的错误发生在哪里。

DROP TABLE IF EXISTS my_table;

CREATE TABLE my_table
(my_int INT NOT NULL);

INSERT INTO my_table VALUES
(1),(2),(3),(4),(5),(6),(7),(8),(9);

SELECT * FROM my_table WHERE my_int BETWEEN 2 AND 8;
+--------+
| my_int |
+--------+
|      2 |
|      3 |
|      4 |
|      5 |
|      6 |
|      7 |
|      8 |
+--------+
7 rows in set (0.00 sec)

SELECT * FROM my_table WHERE my_int BETWEEN 2 AND 4
UNION ALL
SELECT * FROM my_table WHERE my_int BETWEEN 4 AND 8;    
+--------+
| my_int |
+--------+
|      2 |
|      3 |
|      4 |
|      4 |
|      5 |
|      6 |
|      7 |
|      8 |
+--------+
8 rows in set (0.00 sec)