Oracle: Finding Primary Keys - PK and NN - database

Question

I am working with a database and was told the Table USERS has a single Primary Key - USERID. Using ALL_CONS_COLS gives me the following:
OWNER | TABLE_NAME | CONSTRAINT_NAME | COLUMN_NAME | POSITION
----------------------------------------------------------------
MONSTER | USERS | USER_ID_PK | USERREF | 2
MONSTER | USERS | USER_ID_NN | USERID
MONSTER | USERS | USER_ID_PK | USERID | 1
This suggests the PK is a composite comprised of: USERREF and USERID.
There are a few things I do not follow:
Why does USERID have both a NOT NULL constraint and also a PK constraint? PK by definition means NN.
Secondly, as USERID has a NN constraint, why does USERREF not have a NN constraint?
Thirdly, why is CONSTRAINT_NAME "suggesting" the PK is USER_ID (USERID), i.e., why is the constraint named after a single column; wouldn't USERS_PK be a better constraint name?
So, ultimately, is it a Composite PK or not?

Answer 1

So, ultimately, is it a Composite PK or not?
Yes. The primary key is defined on two columns together. The position will tell you the leading column. USER_ID_PK primary key constraint is defined on USERID as leading column followed by USERREF.
Why does USERID have both a NOT NULL constraint and also a PK constraint?
Because someone created a NOT NULL constraint on the primary key column explicitly.
Let's test and see.
Single primary key
SQL> create table t(a number primary key);
Table created.
SQL> SELECT a.table_name,
2 b.column_name,
3 a.constraint_type,
4 b.position
5 FROM user_constraints a
6 JOIN user_cons_columns b
7 ON a.owner = b.owner
8 AND a.constraint_name = b.constraint_name
9 AND a.table_name = b.table_name
10 AND a.constraint_type IN ('P', 'C');
TABLE_NAME COLUMN_NAM C POSITION
---------- ---------- - ----------
T A P 1
Composite primary key
SQL> drop table t purge;
Table dropped.
SQL> create table t(a number, b number);
Table created.
SQL> alter table t add constraint t_pk PRIMARY KEY(a,
Table altered.
SQL> SELECT a.table_name,
2 b.column_name,
3 a.constraint_type,
4 b.position
5 FROM user_constraints a
6 JOIN user_cons_columns b
7 ON a.owner = b.owner
8 AND a.constraint_name = b.constraint_name
9 AND a.table_name = b.table_name
10 AND a.constraint_type IN ('P', 'C');
TABLE_NAME COLUMN_NAM C POSITION
---------- ---------- - ----------
T A P 1
T B P 2
Primary key and NOT NULL
SQL> drop table t purge;
Table dropped.
SQL> create table t(a number primary key not null);
Table created.
SQL> SELECT a.table_name,
2 b.column_name,
3 a.constraint_type,
4 b.position
5 FROM user_constraints a
6 JOIN user_cons_columns b
7 ON a.owner = b.owner
8 AND a.constraint_name = b.constraint_name
9 AND a.table_name = b.table_name
10 AND a.constraint_type IN ('P', 'C');
TABLE_NAME COLUMN_NAM C POSITION
---------- ---------- - ----------
T A C
T A P 1