如何创建一个也允许空值的唯一约束？

问题：

I want to have a unique constraint on a column which I am going to populate with GUIDs. 我想在要用GUID填充的列上具有唯一约束。 However, my data contains null values for this columns. 但是，我的数据包含此列的空值。 How do I create the constraint that allows multiple null values? 如何创建允许多个空值的约束？

Here's an example scenario . 这是一个示例方案。 Consider this schema: 考虑以下模式：

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)
)

Then see this code for what I'm trying to achieve: 然后查看此代码以了解我要实现的目标：

-- 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);

The final statement fails with a message: 最后一条语句失败，并显示一条消息：

Violation of UNIQUE KEY constraint 'UQ_People_LibraryCardId'. 违反UNIQUE KEY约束'UQ_People_LibraryCardId'。 Cannot insert duplicate key in object 'dbo.People'. 无法在对象“ dbo.People”中插入重复密钥。

How can I change my schema and/or uniqueness constraint so that it allows multiple NULL values, while still checking for uniqueness on actual data? 如何更改架构和/或唯一性约束，以便允许多个NULL值，同时仍检查实际数据的唯一性？

解决方案：

参考一： https://stackoom.com/question/3DhZ/如何创建一个也允许空值的唯一约束
参考二： https://oldbug.net/q/3DhZ/How-do-I-create-a-unique-constraint-that-also-allows-nulls

来源：oschina

链接：https://my.oschina.net/u/4438370/blog/4405857

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