如何使复合键成为唯一的? [英] How to make a composite key to be unique?
问题描述
如果您不喜欢阅读跳转到简短部分
问题是我对这种设计不满意。我想将等级
,路径
和 id_class
的组合是独一无二的,作为学生表的主要关键。
我可以删除 student_id
,并从3中复合键,但我也不希望。也许我应该让另一张表让我们说 combination_id
其中等级
,路基
和 id_class
是外键,还有一个额外的列 comb_id
用作表的ID。所有的列都将是主键。但问题是这3列仍然可以重复,因为额外的列( comb_id
)。例如,我可以拥有相同的等级
,路径
和 class_id
但不同的 comb_id
,这将使该行有效,因为表的4列的复合键( combination_id
)
简而言之,我想要 students_id
保持表的唯一主键,但是成为外键另外一张表是等级
,路基
和 class_id
。
如果我不清楚,请在下面的评论中提前谢谢。
PS 对于这个难以置信的标题我很抱歉,但我不太命名
编辑1:
要更清楚:
等级
可以是1到12
路基
可以可以是一个j
id_class
可以是1到30,这是你在课上的号码
所以一个学生可以从7b班和他的班级中的人数 - 5
不要混合独特的概念键和主键。您可以很好地添加一个唯一键,跨越三个等级
,路径
和 class_id
。这样,可能没有两行这三列相同的值。当你写的时候,你不想把这三个作为一个复合的主键,我不知道一个复合的唯一的补充键是否会更好。如果没有,您必须澄清复合键是否可以接受。
要创建唯一键,可以使用以下SQL 声明:
ALTER TABLE学生ADD UNIQUE gsc(成绩,路基,class_id);
单词 gsc
只有一个名字我从主要专栏的缩写组成;使用您想要的任何名称,因为除非您想要识别某些 EXPLAIN
输出或类似的密钥,否则几乎不起作用。
I am making a database of students in one school.Here is what I have so far:
If you don't like reading jump to the "In short" part
The problem is that I'm not happy with this design. I want the combination of grade
, subgrade
and id_class
to be unique and to serve as a primary key for the students table.
I can remove the student_id
and make a composite key from the 3 but I don't want that either. Maybe I should make another table lets say combination_id
where grade
, subgrade
and id_class
are foreign keys and there is one extra column comb_id
that serves as ID for the table. And all the columns will be Primary Keys. But the problem is that those 3 columns can still repeat because of that extra column (comb_id
). For example I can have the same grade
, subgrade
and class_id
but different comb_id
which will make the row valid because of the composite key of the 4 columns of the table (combination_id
).
In short I want students_id
to remain the only primary key of the table but to be a foreign key to another table which is somehow unique combination of grades
, subgrade
and class_id
.
If I was not clear enough ask in the comments below and thank you in advance.
PS I'm sorry for the indescriptive title but I'm bad at naming
EDIT 1:
To be more clear:
grade
can be 1 to 12
subgrade
can be a to j
id_class
can be 1 to 30 and it is your number in class
So a student can be from 7b class and his number in class - 5
Don't mix the concepts of unique keys and primary keys. You can very well add a unique key spanning the three columns grades
, subgrade
and class_id
. That way, no two rows could have the same values for these three columns. As you write that you don't want to have these three as a composite primary key, I'm not sure whether a composite unique supplemental key would be any better. If not, you'll have to clarify when composite keys are acceptable.
To create a unique key, you can use the following SQL statement:
ALTER TABLE students ADD UNIQUE gsc (grades, subgrade, class_id);
The word gsc
there is just a name I made up from the initials of the key columns; use whatever name you want, as it hardly matters unless you want to identify the key in some EXPLAIN
output or similar.
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