Oracle Database (ORA-02270: unique or primary key mismatch for this list) Problems

Oracle Database (ORA-02270: the unique or primary key mismatch for this list) problem was just encountered when writing an oracle database. You cannot find the answer you want on the Internet. I keep thinking about these words. Then, remind me by friends in the group. It turns out that foreign key tables lack unique constraints. Example: www.2cto.com foreign key table: SQL code create table users (uids integer not null, username nvarchar2 (20) unique not null, passwords nvarchar2 (30) not null, udatatime timestamp default sysdate, udescriptions nvarchar2 (255) null); primary table SQL code create table roleUsers (ruid integer primary key, rid int not null, uids int not null, rudescriptions nvarchar2 (255) null, constraint fk_roleUsers_users foreign key (uids) ref Erences users (uids), constraint fk_roleUsers_role foreign key (rid) references role (rid); If the primary key is not added to the foreign key table (unique constraint ), when the primary table references a foreign key, the following error is reported: "ORA-02270: unique or primary key mismatch in this list!