Indexes can be unique or nonunique. Unique indexes guarantee that no two rows of a
table have duplicate values in the key column (or columns). Nonunique indexes do
not impose this restriction on the column values.
Oracle recommends that unique indexes be created explicitly, using CREATE UNIQUE
INDEX. Creating unique indexes through a primary key or unique constraint is not
guaranteed to create a new index, and the index they create is not guaranteed to be a
unique index.

唯一索引和非唯一索引
1. 在一个表上建立唯一索引能够保证此表的索引列不存在重复值.
2. Oracle 建议使用 CREATE UNIQUE INDEX 语句显式地创建唯一索引.