Allow null in unique column

Question

I\'ve created the following table:

CREATE TABLE MMCompany (
   CompanyUniqueID BIGSERIAL PRIMARY KEY NOT NULL, 
   Name VARCHAR (150) NOT NULL,
   PhoneNumb         


        

Answer 1

Some databases do not allow multiple null values, for example the SQL Server documentation states that "multiple null values are considered duplicates". On databases that do not allow nullable UNIQUE constraints you could try this (from GuidoG's answer to another question):

CREATE UNIQUE NONCLUSTERED INDEX IDX_Email
ON MMCompany (Email)
WHERE Email IS NOT NULL;