Question: I know that `UNIQUE` can be used for unique value on table creating.

I read in a database management book that 

> 
> When we apply UNIQUE to a subquery, the
>  resulting condition returns true if no row appears twice in the answer to the
>  subquery that is, there are no duplicates; in particular, it returns true if the
>  answer is empty.
> 
> 
> 

I didn't see any query like that ,Is it possible?

Answer: 
`UNIQUE` is defined by *SQL92* Section 8.9: `<unique predicate> ::= UNIQUE <table subquery>` so it certainly exists but is not widely supported by vendors.

As an alternative you could use EXISTS with a HAVING COUNT.
The UNIQUE constraint is typically used for defining table based constraints.

You should consider using `Select Distinct` for queries. 

The following link provides details regarding the `Unique` and `Distinct` keywords.

<http://psoug.org/definition/DISTINCT.htm>