问我如何创建一个允许空值的唯一约束？

CREATE TABLE People (

  Id INT CONSTRAINT PK_MyTable PRIMARY KEY IDENTITY,

  Name NVARCHAR(250) NOT NULL,

  LibraryCardId UNIQUEIDENTIFIER NULL,

  CONSTRAINT UQ_People_LibraryCardId UNIQUE (LibraryCardId)

)

-- This works fine:

INSERT INTO People (Name, LibraryCardId) 

 VALUES ('John Doe', 'AAAAAAAA-AAAA-AAAA-AAAA-AAAAAAAAAAAA');

-- This also works fine, obviously:

INSERT INTO People (Name, LibraryCardId) 

VALUES ('Marie Doe', 'BBBBBBBB-BBBB-BBBB-BBBB-BBBBBBBBBBBB');

-- This would *correctly* fail:

--INSERT INTO People (Name, LibraryCardId) 

--VALUES ('John Doe the Second', 'AAAAAAAA-AAAA-AAAA-AAAA-AAAAAAAAAAAA');

-- This works fine this one first time:

INSERT INTO People (Name, LibraryCardId) 

VALUES ('Richard Roe', NULL);

-- THE PROBLEM: This fails even though I'd like to be able to do this:

INSERT INTO People (Name, LibraryCardId) 

VALUES ('Marcus Roe', NULL);