Why to use "not null primary key" in TSQL?

Answer 1

Edited for clarity

According to the SQL Specification, a primary key can not contain NULL. This means that decorating a column with either "NOT NULL PRIMARY KEY" or just "PRIMARY KEY" should do the same thing. But you are relying on the SQL engine in question correctly following the SQL standard. As an example due to an early bug, SQL Lite does not correctly implement the standard and allows null values in non-integer primary keys (see sqlite.org/lang_createtable.html). That would mean for (atleast) SQLLite the two statements do two different things.

As "NOT NULL PRIMARY KEY" clearly indicates intent and continues to enfore non-null values even if the primary key is removed, that should be the prefered syntax.

Answer 2

It isn't required syntactically as this will be the default anyway if not specified for Primary Key columns regardless of the default setting for the other columns.

SET ANSI_NULL_DFLT_ON ON /*Set default for columns to allow NULL if not specified*/

CREATE TABLE #t
(
i INT PRIMARY KEY,
j INT
)

SELECT name,is_nullable
FROM tempdb.sys.columns 
WHERE object_id=object_id('tempdb..#t')


INSERT INTO #t VALUES (1,NULL) /*Succeeds as j allows NULL*/

INSERT INTO #t VALUES (NULL, 1)/*Won't succeed as column doesn't allow nulls*/

DROP TABLE #t

Answer 3

All primary key means it is the identifier for the table and therefor unique by definition. Unique doesn't mean not null, so if you have the requirement that the primary key can't be null you have to specify so explicitly. If you allow null you're just saying that it is ok to have a null value in the column, but you're only allowed to have one since it has to be unique.

Answer 4

The following:

CREATE TABLE T1 (
    T1ID int not null primary key
)

is really shorthand for the following:

CREATE TABLE T1 (
    T1ID int not null,
    constraint <system generated name> PRIMARY KEY (T1ID) on [PRIMARY]
)

The column definition is really quite separate from the constraint definition. It's just that the shorthand hides the fact that it's two separate definitions. (Of course, if you use the long-hand form, you can do such things as defining a primary key across multiple columns).

Edit, in response to update. No, you still cannot have a nullable column in the primary key. But given that it's normal to specify the nullability of each column in a create table statement, it just feels odd to omit it (even though it's implied/required by the PRIMARY KEY constraint).

CREATE TABLE T1 (
    T1ID int,
    constraint <system generated name> PRIMARY KEY (T1ID) on [PRIMARY]
)

creates exactly the same table as the previous code sample. Explicitly specifying the "not null"-ability of a column is, then, more an explicit acknowledgement of the outcome, rather than a requirement.

And finally, you'll find it frequently occurring in example code because people have developed their database, and then used the built in SQL tools for generating a script from their database - the scripting tools always put everything explicitly.

Answer 5

I don't know of any DBMS that permits a nullable column to be used in a PRIMARY KEY constraint. The SQL standard specifies that columns in a PRIMARY KEY constraint cannot be nullable.

In the relational model candidate keys consist of unique values, not nulls. It is a peculiarity of SQL that nullable columns can optionally be used in UNIQUE constraints. That feature is probably responsible for a lot of data quality problems. I highly recommend you avoid using nullable columns in UNIQUE constraints. If necessary, make a new table for the non-nullable values and put the UNIQUE constraint on that.