I have the table Tester on oracle with the following columns:

* TesterID
* TesterName
* IsDefault
* Application\_ID

TesterID is the primary key. 
Now I want that there can only be one Default Tester, which means only one Tester can have the calues IsDefault =Y at an ApplicationID.

I tried it with a constraint:

```
alter table Tester add constraint Tester_ISDEFAULT UNIQUE(IsDefault,Application_ID);

```

Is it possible to make the unique key on where isdefault= Y?

Thanks for help!