I have the table Tester on oracle with the following columns:
alter table Tester add constraint Tester_ISDEFAULT UNIQUE(IsDefault,Application_ID);
Not with a
UNIQUE constraint. However, you can use a
UNIQUE INDEX instead:
CREATE UNIQUE INDEX ApplicationId_Default_Y ON tester ( CASE WHEN IsDefault = 'Y' THEN ApplicationId ELSE NULL END );
Here's a DEMO.