Do we need to specify “not null” for primary key? Oracle/SQL [closed]

Answer 1

create table mytable (
  col1 number primary key,
  col2 number,
  col3 number not null
);

table MYTABLE created.

select table_name, column_name, nullable 
from user_tab_cols where table_name = 'MYTABLE';

TABLE_NAME                     COLUMN_NAME                    NULLABLE
------------------------------ ------------------------------ --------
MYTABLE                        COL1                           N        
MYTABLE                        COL2                           Y        
MYTABLE                        COL3                           N        

So, no, you do not need to specify primary key columns as NOT NULL.

Answer 2

Primary key by definition can never be Null. Primary key purpose is to uniquely identify records. A primary key is a combination of columns which uniquely specify a row.

A Null value represents lack of value. Even if two records have NULL in same column , the column values are not considered equal.

Answer 3

In most DBMS, since its a primary key (and the definition of is that it must be unique within the table) then it most certainly cannot be null.

Answer 4

Yes, as @eaolson said, you don't need to specify NOT NULL for primary key columns, they are set automatically to NOT NULL.

However, Oracle keeps track that you didn't specify NOT NULL explicitly in case the primary key is disabled or dropped later on:

create table mytable (
  col1 number,
  col2 number not null
);

select table_name, column_name, nullable
  from user_tab_columns where table_name = 'MYTABLE';

TABLE_NAME   COLUMN_NAME  NULLABLE
------------ ------------ ---------
MYTABLE      COL1         Y
MYTABLE      COL2         N

As expected, col1 is nullable and col2 NOT NULL. A primary key changes both columns to NOT NULL:

alter table mytable add primary key (col1, col2);

select table_name, column_name, nullable
  from user_tab_columns where table_name = 'MYTABLE';

TABLE_NAME   COLUMN_NAME  NULLABLE
------------ ------------ ---------
MYTABLE      COL1         N
MYTABLE      COL2         N

If you disable or drop the primary key, both columns revert to the original state, co1 becomes nullable again:

alter table mytable disable primary key;

select table_name, column_name, nullable
  from user_tab_columns where table_name = 'MYTABLE';

TABLE_NAME   COLUMN_NAME  NULLABLE
------------ ------------ ---------
MYTABLE      COL1         Y
MYTABLE      COL2         N