我想在一个列上有一个唯一的约束，我将用guid填充这个列。但是，我的数据包含这些列的空值。如何创建允许多个空值的约束?
下面是一个示例场景。考虑这个模式:
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)
)
然后看看我想要实现的代码:
-- 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);
最后一个语句失败，并返回一条消息:
违反唯一键约束'UQ_People_LibraryCardId'。不能在对象'dbo.People'中插入重复的键。
我如何改变我的模式和/或唯一性约束，使它允许多个NULL值，同时仍然检查对实际数据的唯一性?
