Oracle SQL Constraint where clause

Question

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!

Answer 1

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.

Answer 2

You can do this with a function-based unique index, not a constraint as such:

create unique index tester_isdefault on tester 
  (case when isdefault='Y' then application_id end);

Since Oracle doesn't created index entries for keys that are all null, only rows where isdefault='Y' will appear in the index.

Answer 3

That constraint won't work as it would mean you can only have two rows for each Application_ID - one with IsDefault=0 and the other with IsDefault=1.

You could enforce this logic with a trigger. Or, why not just enforce it in your application logic?

Answer 4

create unique index tester_ui_1 on tester(decode(is_default, 'Y', 0, tester_id), application_id)