php - 从学生表中获取 enrolment_id 为空的记录

Question

enrolment_id是student表中的外键，我想获取尚未设置外键的学生。含义（但这在子句enrolment_id = NULL中不起作用）。WHERE

该怎么办？

Answer 1

比较NULL值时，请改用IS NULL谓词：

...
WHERE enrolment_ID IS NULL;

= NULL不会工作。

Answer 2

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