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

Unique and null don't get along much, since null is undefined by definition — you can't know if two nulls are the same unknown.

In this sense, your current unique constraint on email is the right thing to do and should work as is.

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In case you ever need to make it otherwise, though, a partial index works:

create unique index on MMCompany((email is null)) where (email is null);

Another approach is to define a constraint trigger. Something like:

create function email_chk() returns trigger as $$
begin
  if exists (
    select 1 from mmcompany where email is null and companyuniqueid <> new.id
  ) then
    raise 'dup null found';
  end if;
  return null;
end;
$$ language plpgsql;

create constraint trigger after insert or update on mmcompany
for each row when (new.email is null)
execute procedure email_chk();