According to the ANSI standards SQL:92, SQL:1999, and SQL:2003, a UNIQUE constraint should disallow duplicate non-NULL values, but allow multiple NULL values.
A Unique Constraint in Vertica does just that!
But what if I do not want to allow more than one NULL value? Well, there’s a simple trick for that!
Read about it here:
https://my.vertica.com/blog/vertica-qui ... onstraint/