Question

我有一个名为Enrollment的数据库，其中所有学生都将添加批处理和课程。 stud_id，course_id和batch_id指的是带有fk的另一个表。

 enrolment
+----------+---------+-----------+----------+
| enrol_id | stud_id | course_id | batch_id |
+----------+---------+-----------+----------+
|        1 |       1 |         2 |        1 |
|        2 |       5 |         3 |        2 |
|        3 |       4 |         5 |        3 |
+----------+---------+-----------+----------+

在我删除student表中的学生记录之前，我想将stud_id表格中的students与stud_id中的enrollment进行比较表。如果匹配，请不要删除学生。或者某种消息说这条记录与登记表或其他东西相关联。

如果匹配，我想运行此$sql = "DELETE FROM students WHERE student_id = $id ";

students
+---------+------------+-----------+
| stud_id | first_name | last_name |
+---------+------------+-----------+
|       1 | John       | Doe       |
|       2 | Susy       | Roberts   |
|       3 | John       | redneck   |
+---------+------------+-----------+

请帮助!!

Answer 1

可以使用NOT IN

来实现

DELETE FROM students
WHERE student_id NOT IN (SELECT stud_id 
                    FROM enrolment) AND student_id = $id

可以使用NOT EXISTS

来实现

DELETE FROM students
WHERE NOT EXISTS(SELECT NULL
                FROM enrolment
               WHERE stud_id = $id) AND student_id = $id

Answer 2

您想要运行类似

的查询

$sql = SELECT COUNT(1) FROM enrolment WHERE stu_id = $id

然后检查内容以获取返回的数字＆gt; 0然后错误。

如何在删除MySQL中的记录之前将数据与另一个表的数据进行比较

2 个答案: