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

I'm not entirely aware of your intended use or your tables, but you could try using a one to one relationship. Split out this "sometimes" unique column into a new table, create the UNIQUE index on that column in the new table and FK back to the original table using the original tables PK. Only have a row in this new table when the "unique" data is supposed to exist.

OLD tables:

TableA
ID    pk
Col1  sometimes unique
Col...

NEW tables:

TableA
ID
Col...

TableB
ID   PK, FK to TableA.ID
Col1 unique index