/* Check the IndepYear field for an empty value. */
SELECT Name
FROM country
WHERE IndepYear IS NULL
/* Select the records for which the
IndepYear field contains the value. */
SELECT Name
FROM country
WHERE IndepYear IS NOT NULL
| Name |
| Aruba |
| Anguilla |
| Netherlands Antilles |
| American Samoa |
| Antarctica |
| French Southern territories |
| Bermuda |
| Bouvet Island |
| Cocos (Keeling) Islands |
| ... |
| 判断值是否为 NULL，不能简单地写 = NULL。为此 SELECT 语句的 WHERE 子句提供了专门的写法：IS NULL。 |