First , let us learn the basics about Nulls in programming,
Null are actually a flag which suggest nonexistent or unknown value.
It is not zero, not an empty string. A value cannot equal to a
NULL even two
NULL values are equal.
NULL !=
NULL .
The Primary Key is a Unique Identifies, according to the definition of the Primary Key, every row has to be unique and can be identified uniquely. And null is unknown value, so the primary key cannot accept
NULL values.
On the top of all,
ANSI standards say a Primary Key can not be NULL.
Good links to follow on NULL:
https://www.simple-talk.com/sql/t-sql-programming/how-to-get-nulls-horribly-wrong-in-sql-server/ http://sqlblog.com/blogs/hugo_kornelis/archive/2007/09/30/what-if-null-if-null-is-null-null-null-is-null.aspx Other Related Question in series: Can We Create Primary key with Clustered Index in SQL Server?
How to create a primary key without Clustered index?
Why SQL Server by default creates clustered index on the primary key?
Can we create a Primary key without clustered index?
Can we create primary key without any index?
Can we create primary key on more than one column?
Can we have a functinality of primary key without creating a primary key?
Why primary key doesn`t accepts null values?
What is the need of primary key?
Is it mandotary to have a primary key in the table?
We will try to answer all the above questions and link all of them to each other,
We welcome everybody to contribute to this question answer series.
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