sql - SQL Server 表上的唯一分组约束

Question

在 T-SQL 中，我遵循一个约束，该约束将阻止为该标志插入重复记录。

对于每个 [FK_Something]，标志 [MyFlag] 只能设置为值 1 一次，但可以多次设置为 0。

这是否可能，并且在不使用触发器的情况下如何实现？

我使用的是 SQL Server 2008 R2。

set nocount on

IF  EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[MyTable]') AND type in (N'U'))
    DROP TABLE [dbo].[MyTable]
GO

CREATE TABLE [dbo].[MyTable]
(
    [id]            [int] IDENTITY(1,1) NOT NULL,
    [FK_Something]  [int] NOT NULL,
    [MyFlag]        [bit] NOT NULL
) ON [PRIMARY]
GO

--  Valid inserts   --
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (1,0);
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (1,1);
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (1,0);
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (2,0);
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (2,0);
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (2,0);
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (2,1);

---------------------------------------
-- Inserts Should Fail  
-- As [MyFlag] set for [FK_Something]
---------------------------------------
INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (1,1)   
GO

INSERT INTO [MyTable]   ([FK_Something],[MyFlag])    VALUES  (2,1)
GO

-- Constraint should apply this logic --
Declare @FK_Something   INT
Declare @MyFlag         INT
set @FK_Something = 2
Set @MyFlag       = 0

if( @myflag = 0 OR
    not exists(Select 1 from [MyTable] where FK_Something = @FK_Something and MyFlag = @MyFlag))
BEGIN
    INSERT INTO [MyTable]   ([FK_Something],[MyFlag])  VALUES  (@FK_Something,@MyFlag);
    print 'Inserted ';
END
GO

select * from [MyTable]

Answer 1

指数。

For each [FK_Something] the flag [MyFlag] can only be set once to value 1 but can be set to 0 many times.

这是 1 个简单索引:

• FK_Something 和 MyFlag 1 - 独特

这里的技巧是不要对所有行应用唯一索引，这就是添加过滤器的原因。

http://msdn.microsoft.com/en-us/library/cc280372.aspx

解释如下语法:

CREATE UNIQUE NONCLUSTERED INDEX IX_Whatever
    ON MyTable (FK_Something, myFlag)
    WHERE MyFlag = 1 

比约束更简单，并且保证有效。