从学生表中获取其enrolment_id为空的记录

Question

enrolment_id是student表中的外键，我希望得到那些尚未设置外键的学生。含义enrolment_id = NULL（但这在WHERE子句中不起作用。）

怎么办？

Answer 1

比较NULL值时，请改用IS NULL谓词：

...
WHERE enrolment_ID IS NULL;

= NULL无效。

Answer 2

SELECT * FROM student AS s WHERE s.enrolment_ID IS NULL;