SQL can I have a “conditionally unique” constraint on a table?

Question

I\'ve had this come up a couple times in my career, and none of my local peers seems to be able to answer it. Say I have a table that has a \"Description\" field which is a

Answer 1

Oracle does. A fully null key is not indexed by a Btree in index in Oracle, and Oracle uses Btree indexes to enforce unique constraints.

Assuming one wished to version ID_COLUMN based on the ACTIVE_FLAG being set to 1:

CREATE UNIQUE INDEX idx_versioning_id ON mytable 
  (CASE active_flag WHEN 0 THEN NULL ELSE active_flag END,
   CASE active_flag WHEN 0 THEN NULL ELSE id_column   END);