How to structure a listings table
There is a normal form that would separate this column into its own entity (table). If you want to go down that path it's a legitimate choice. In my experience it's rare, though.
Accounting for unknown values is part of life and business computing. Procedural languages have their techniques. SQL has nulls. One way or another you'll eventually have to deal with nulls in the system, even if all table columns are defined &quot;NOT NULL&quot;. Outer joins, for example, may introduce them.
As for performance, RDBM systems have efficient mechanisms to deal with nulls so this should not be a concern.
Were I writing this I would add the nullable column to the existing table.