one-to-many,one-to-one and many-to-many associations

create table USERS (
    USER_ID bigint not null primary key,
    USERNAME varchar(15) not null unique,
    NAME varchar(50) not null,
...
)

create table BILLING_DETAILS (
    BILLING_DETAILS_ID bigint not null primary key,
    ACCOUNT_NUMBER VARCHAR(10) not null unique,
    ACCOUNT_NAME VARCHAR(50) not null,
    ACCOUNT_TYPE VARCHAR(2) not null,
    USER_ID bigint foreign key references USERS
)

 

This is one-to-many associations:

Table associations, on the other hand, are always one-to-many or one-to-one. You can
see the multiplicity immediately by looking at the foreign key definition. The following
is a foreign key declaration on the BILLING_DETAILS table for a one-tomany
association (or, if read in the other direction, a many-to-one association):

USER_ID bigint foreign key references USERS

 

These are one-to-one associations:

 

USER_ID bigint unique foreign key references USERS

BILLING_DETAILS_ID bigint primary key foreign key references USERS

 

the two are equals.

 

many-to-many associations:

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 domain model. For our example, if we consider the relationship between
the user and the 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 USERS,
    BILLING_DETAILS_ID bigint foreign key references BILLING_DETAILS,
    PRIMARY KEY (USER_ID, BILLING_DETAILS_ID)
)