Question

I have a situation where I need to enforce a unique constraint on a column[attribute] depending on another column value.

So for example, I have a table like Table(ID, EID, Name, ISDeleted)

ISDeleted can only have a value null or ‘y’ (active or deleted), and i want to create a unique constraint on EID, ISDeleted only when ISDeleted = null, since I dont care if there are multiple deleted records with the same id. Please note that, EID can have null value.

I am using Oracle DB for this.

Answer 1

You can’t create a constraint. But you can create a unique function-based index. This takes advantage of the fact that Oracle does not index NULL values– any rows where isDeleted is NOT NULL will not be included in the index so the unique constraint won’t apply to them.

CREATE UNIQUE INDEX one_not_deleted
    ON table_name( (CASE WHEN isDeleted IS NULL
                         THEN eid
                         ELSE null
                      END) );