在 SQL Server 中实现允许多个 NULL 值的唯一约束的正确方法 [英] The proper way to implement unique constraint that allows multiple NULL values in SQL Server

问题描述

我需要表中的 1 列来保存唯一的非空值或 NULL.TSQL UNIQUE 约束将 2 个 NULL 视为相等，因此我无法使列唯一.
处理这个问题的正确方法是什么?
经过一番研究，我发现了两种对我来说似乎正确的方法，但我无法确定哪一种更好.
第一个不适用于所有情况:

I need 1 column in the table to hold unique non-null values or NULL. TSQL UNIQUE constraint treats 2 NULLs as equal, so I cannot make column unique.
What is the right way to handle this problem?
After doing some research, I found 2 methods which seem correct to me, but I cannot determine which one is better.
The first which is not applied to all cases:

CREATE TABLE test (id INT NOT NULL IDENTITY(1,1) PRIMARY KEY, 
   null_or_unique_id INT, unique_key AS  
(CASE WHEN [null_or_unique_id] IS NULL THEN -(1)*[id] 
 ELSE [null_or_unique_id] END), UNIQUE(unique_key ));

它可以工作，但要求 null_or_unique_id 的所有允许值都为非负(在我的情况下没问题)
第二个:

It works but requires all allowed values of null_or_unique_id to be non-negative (that's ok in my case)
The second one :

 CREATE VIEW test_view WITH SCHEMABINDING AS
 SELECT [null_or_unique_id] FROM dbo.test WHERE [null_or_unique_id] IS NOT NULL;
 GO
 CREATE UNIQUE CLUSTERED INDEX byNullOrUniqueId 
 ON dbo.test_view([null_or_unique_id]);

当然，也可以使用触发器来实现所需的功能，但我认为触发器解决方案会产生比上述任何一种都更多的开销.

Surely, it's also possible to implement the desired functionality with triggers, but I think trigger solution will create more overhead then any of mentioned above.

这种情况的最佳实践是什么?
感谢您的回答.

What is the best practice for such a case?
Thanks for your answers.

推荐答案

4 种方式:

• 过滤索引 (SQL Server 2008) <- 基于推荐在您的标签上
• 触发器(提及)
• 索引视图(在您的问题中)
• 具有计算列的唯一约束/索引(在您的问题中)

• Filtered index (SQL Server 2008) <- recommended based on your tags
• Trigger (mentioned)
• Indexed view (in your question)
• Unique constraint/index with computed column (in your question)

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