If you wish to represent a many-to-many association in a relational database, you must introduce a new table, called a link table. This table doesn’t appear anywhere in the object model. For our example, if we consider the relationship between a user and the user’s billing information to be many-to-many, the link table is defined as follows:

CREATE TABLE USER_BILLING_DETAILS (
USER_ID BIGINT FOREIGN KEY REFERENCES USER,
BILLING_DETAILS_ID BIGINT FOREIGN KEY REFERENCES BILLING_DETAILS
PRIMARY KEY (USER_ID, BILLING_DETAILS_ID)
)