Does a Join table (association table) have a primary key ? many to many relationship

Question

Does a Join table (association table) have a primary key ? many to many relationship. I've seen some Join tables with a primary key and some without can someone please explain when you would have a primary key in a join table and why?

Thank you in advance;-)

Answer 1

In a pure 'join' or junction table all the fields will be part of the primary key. For example let's consider the following tables:

CREATE TABLE USERS
  (ID_USER NUMBER PRIMARY KEY,
   FIRST_NAME VARCHAR2(32),
   LAST_NAME VARCHAR2(32));

CREATE TABLE ATTRIBUTES
  (ID_ATTRIBUTE NUMBER PRIMARY KEY,
   ATTRIBUTE_NAME  VARCHAR2(64));

A junction table between these to allow many users to have many attributes would be

CREATE TABLE USER_ATTRIBUTES
  (ID_USER NUMBER REFERENCES USERS(ID_USER),
   ID_ATTRIBUTE NUMBER REFERENCES ATTRIBUTES(ID_ATTRIBUTE),
   PRIMARY KEY(ID_USER, ID_ATTRIBUTE));

Sometimes you'll find the need to add a non-primary column to a junction table but I find this is relatively rare.

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