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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) ) |
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