One of the least known, least used features of SQL is the CREATE DOMAIN statement. It does not exist in SQL Server as of SQL Server 2022, but it has been a part of PostgreSQL since 7.3.
Some definitions from other smart people
Chris Date defined a domain as “a pool of values from which the actual values appearing in a column (relational table) may be drawn. This is really weaker than it needs to be. For example, this definition would let a list of possible integers greater than zero, which are used model the distance domain.
Terry Halpern defined