How to create unique index on fields with possible null values (Oracle 11g)?

Question

Here is the sample table with 3 columns (ID, UNIQUE_VALUE, UNIQUE_GROUP_ID)

I want below records can be allowed:

(1, NULL, NULL)
(2, NULL, NULL)

or

(3, NULL, 7)
(4, 123, 7)

or (Note: this condition is not allowed in unique index nor unique constraint)

(5, NULL, 7)
(6, NULL, 7)

and these can't be allowed:

(7, 123, 7)
(8, 123, 7)

I created a unique index on last 2 columns, but only the first 2 examples can be allowed.

Is it possible to let db check the uniqueness of these 2 columns only when both are not null?

Answer 1

You want to only enforce uniqueness on the rows where both UNIQUE_VALUE and UNIQUE_GROUP_ID are not null. To do this, you can use a unique function-based index:

CREATE UNIQUE INDEX func_based_index ON the_table
  (CASE WHEN unique_value IS NOT NULL
         AND unique_group_id IS NOT NULL
        THEN UNIQUE_VALUE || ',' || UNIQUE_GROUP_ID
   END);