Can key constraint application in Database be applied on a part of a single column?

As we know in data bases Key Constraints is the most important concept.
we can apply the key constraint either to column level or table level, but
I want to know is there any possibility that a unique key be applied only on to a part of a single column?
I was asked this question by one of my friends, as far as i know there is no concept like that.

example there is a column named timestamp and value in that column is like 2014-06-23 01:02:03
so i want to apply unique key based on time only, ignoring the date.
is that possible?

So please help me know if there is any possibility. Posted in: #Coffee Room

Technically not directly. Ull have to define a rule where the characters preceding the checkarea n following it would have to be masked. So ur basically splitting the value.

Another approach can be through checksum

thank you Nayan 😀
but can you be more precise?
i am talking about databases.
i am not aware about masking thing, can u help me know how?

No you can't, at database level

ok thank you