Question: Is it required to make foreign key column in a table as NOT NULL,
If we not written explicitly foreign key column as not null what it will be?
Can it contain null values?

And what is the Difference between following two statements:

```
[PhoneId] [int] NOT NULL FOREIGN KEY REFERENCES [dbo].[tbl_PhoneNumber](PhoneNumberId)
[PhoneId] [int] FOREIGN KEY REFERENCES [dbo].[tbl_PhoneNumber](PhoneNumberId)

```

Answer: 
According to sql normalization rule foreign key value must be equal to primary key value or NULL, so it will contain either NULL value of one value from primary key table row.
It is not required. A foreign key attribute without `NOT NULL` can contain `NULL` values, and this can be used to indicate that no such tuple in the referenced relation is applicable.