你可能不知道的SQL问题

1.  如下是sql语句和结果，

SELECT count(*) AS total FROM orders;

+-------+
| total |
+-------+
|  150  |
+-------+

SELECT count(*) AS cust_123_total FROM orders WHERE customer_id = '001';

+----------------+
| cust_123_total |
+----------------+
|       65       |
+----------------+

请分析下面SQL的执行结果：

SELECT count(*) AS cust_not_123_total FROM orders WHERE customer_id <> '001'

解析： 第一眼看上去觉得很简单吧， 这个结果就是85嘛， 但是实际上可能不是， 因为customer_id <> '001' 这个条件不包含customer_id值为null的情况，也就是说这个结果是小于等于85.

2. 来看下一个，你可以直接分析出来吗？

sql> SELECT * FROM runners;
+----+--------------+
| id | name         |
+----+--------------+
|  1 | John Doe     |
|  2 | Jane Doe     |
|  3 | Alice Jones  |
|  4 | Bobby Louis  |
|  5 | Lisa Romero  |
+----+--------------+

sql> SELECT * FROM races;
+----+----------------+-----------+
| id | event          | winner_id |
+----+----------------+-----------+
|  1 | 100 meter dash |  2        |
|  2 | 500 meter dash |  3        |
|  3 | cross-country  |  2        |
|  4 | triathalon     |  NULL     |
+----+----------------+-----------+
下列语句的结果是什么?

SELECT * FROM runners WHERE id NOT IN (SELECT winner_id FROM races)

解析：