unique constraint

基本解释

The unique constraint is created in the database when you save the table.
当保存表时，将在数据库中创建该唯一约束。
This clause is not allowed if the column is specified in the primary key, or in a unique constraint of the table (SQLSTATE 42831).
如果表的主键或惟一性约束中指定了这个列，那么不允许使用这个子句（SQLSTATE 42831）。
The column 'CITY' in the OFFICE_INFO nickname is unique and hence a unique constraint has been defined on the nickname OFFICE_INFO on column CITY as follows
OFFICE_INFO 昵称中的列 'CITY' 是惟一的，因此在列 CITY 上定义了昵称 OFFICE_INFO 的惟一约束，如下所示

The twenties, possibly alone among the middle decades of life, are passionately celebrated in fiction and memoirs, and the celebrations tend to share a style that is personal, specific, cliquish, pastiched, breathless, often bibulous, and flagrantly confessional—the voice of early mastery without mature constraint, self-discovery at a moment when each revelation seems unique.
NEWYORKER: Semi-Charmed Life