如何在表格中强制执行唯一性? [英] How do I enforce uniqueness in a table?

问题描述

例如，我必须编写一个具有以下内容的注册表:

For example, I have to program an enrolment table which has:

1. enrollment_id
2. academic_period
3. 结果
4. student_id
5. 主题

我想强制执行一项操作，以确保学生在特定学期内不能多次注册该科目. 因此，约束条件应包括学生ID"，学科"和学期"列.

I want to enforce to make sure that a student can't enroll in a subject more than once in a specific academic period. So, the constraint shall involve student_id, subject, and academic period columns.

我仍然不确定如何做到这一点.有帮助吗?

I'm still unsure how do I make this happen. Any help?

推荐答案

向表中添加唯一约束:

ALTER TABLE yourtable ADD CONSTRAINT constraintname UNIQUE (student_id, subject, academic_period)

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