oracle创建多对多关系的SQL语句

create table user_  (
   user_id               number(8,0)                   not null,
   user_password        VARCHAR2(50),
   user_name            VARCHAR2(50),
   constraint PK_USERS primary key (user_id)
);

create table role_ (
   role_id               number(8,0)                    not null,
   role_name            VARCHAR2(20),
   constraint PK_ROLES primary key (role_id)
);

create table user_role  (
   role_id               number(8,0)                   not null,
   user_id               number(8,0)                    not null,
   constraint PK_USER_RELATE_ROLE primary key (role_id, user_id),
   constraint fk_1 FOREIGN KEY(role_id) REFERENCES role_(role_id),
   constraint fk_2 FOREIGN KEY(user_id) REFERENCES user_(user_id)
);