Why primary key cannot be null

See more: SQL-Server. Hi, in my recent interview I have been asked why primary key can't be null?? Posted Jan am Rakesh Agarwal Add a Solution. Top Rated Most Recent. Accept Solution Reject Solution. What is the fundamental attribute required of a Primary Key?

So, how unique is NULL? Allowing a primary key to contain nulls would cause a potential conflict with the uniqueness constraint. Posted Jan am OriginalGriff. Technically, a primary key constraint is simply a combination of a unique constraint and a not-null constraint.

A primary key indicates that a column or group of columns can be used as a unique identifier for rows in the table. This is a direct consequence of the definition of a primary key. Note that a unique constraint does not, by itself, provide a unique identifier because it does not exclude null values. This is useful both for documentation purposes and for client applications. If you try to create the Constraints in SQL again, it will be created successfully as shown below:.

Beside each column, you will find a small checkbox that you can use to specify the null-ability of that column. In other words, the index that is automatically created when you define a UNIQUE constraint will guarantee that no two rows in that table can have the same value for the columns participating in that index, with the ability to insert only one unique NULL value to these columns, if the column allows NULL.

Let us create a small table with two columns, ID and Name. The first two records will be inserted successfully, with no constraint preventing duplicate values of the Name column. The constraint in SQL creation will fail, due to having duplicate values of that column in the table, as shown in the error message below:. You can also see the automatically created index that is used to guarantee the column values uniqueness.

Right-click on the needed table and choose Design. Save Article. Improve Article. Like Article. Previous Difference between Clustered and Non-clustered index. Recommended Articles. Print all unique digits present in concatenation of all array elements in the order of their occurrence. Download Microsoft Edge More info.

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