质数属性可以有空值吗?

I understand that a candidate key can not have NULL values. But a candidate key can itself be a combination of many attributes which are called prime attributes. Can these prime attributes have NULL as a value?

我理解一个候选键不能有空值。但是候选键本身可以是许多属性的组合，这些属性称为素数属性。这些素数属性作为值是否为NULL ?

Regards,

问候,

2 个解决方案

#1

Prime attribute An attribute which appears as an attribute for some candidate key. (It needn't be the primary key, or even any key of interest.)

属性:作为某个候选键的属性出现的属性。(它不必是主键，甚至是任何感兴趣的键。)

Schemas and keys

模式和钥匙

Prime attributes are part of a candidate key. A candidate key, as the name suggests, is a candidate for primary key. That means, a candidate key can become a primary key, if required.

基本属性是候选键的一部分。顾名思义，候选键是主键的候选键。这意味着，如果需要，候选键可以成为主键。

Now if you recall, any primary key cannot be null. So, by definition, a prime attribute cannot be null. If it is null, it ceases to be a prime attribute.

如果你还记得，任何主键都不能为空。根据定义，一个素数属性不能为空。如果它是null，它就不再是一个prime属性。

Here is an explanation.

这是一个解释。

If you have a composite candidate key lets say

如果你有一个复合候选键

CK1(P1, P2)

P1 and P2 are a prime attributes. You can set P1 or P2 as null, because there is no constraint on prime attributes other than those of primary key attributes.

P1和P2是质数属性。您可以将P1或P2设置为null，因为除了主键属性之外，对主要属性没有约束。

So,

所以,

CK1(P1, P2=null)

Now, CK1 cannot be a primary key, hence is no more a candidate key and hence, P2 is no more a prime attribute.

现在，CK1不能是主键，因此不再是候选键，因此P2不再是质数属性。

#2

I believe it depends on the database you use.

我相信这取决于你使用的数据库。

For example, you don't have to do it explicitly in MySQL. This is defined in the manual:

例如，不需要在MySQL中显式地执行。这在手册中有规定:

A PRIMARY KEY is a unique index where all key columns must be defined as NOT NULL. If they are not explicitly declared as NOT NULL, MySQL declares them so implicitly (and silently).

主键是唯一的索引，其中所有键列必须定义为NOT NULL。如果它们没有被显式声明为非空，那么MySQL将它们隐式地(并且是静默地)声明。

#1