Sql server 如何在SQL Server中为表创建复合候选键？ 请考虑下表定义…

表格定义

CREATE TABLE [dbo].[Folders](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [UserId] [int] NOT NULL,
    [ParentFolderId] [int] NULL
PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[Folders]
ADD CONSTRAINT AK_Folders_UserId UNIQUE ([UserId], [Id])
GO

其他表与此表的主键列具有外键关系

[id]

我希望添加一个自引用外键约束，其中父文件夹id引用同一文件夹表中另一条记录的id字段，但用户id也必须匹配

自引用外键约束

ALTER TABLE [dbo].[Folders]  WITH CHECK ADD  CONSTRAINT [FK_Folders_ParentFolder] FOREIGN KEY([UserId], [ParentFolderId])
REFERENCES [dbo].[Folders] ([UserId], [Id])
GO
ALTER TABLE [dbo].[Folders] CHECK CONSTRAINT [FK_Folders_ParentFolder]
GO

。。。但是我有错误

错误

Msg 1776, Level 16, State 0, Line 64
There are no primary or candidate keys in the referenced table 'dbo.Folders' that match the referencing column list in the foreign key 'FK_Folders_ParentFolder'.
Msg 1750, Level 16, State 0, Line 64
Could not create constraint or index. See previous errors.
Msg 4917, Level 16, State 0, Line 67
Constraint 'FK_Folders_ParentFolder' does not exist.
Msg 4916, Level 16, State 0, Line 67
Could not enable or disable the constraint. See previous errors.

我曾尝试向表中添加一个复合唯一索引以提供候选键，但这不起作用，我仍然收到相同的错误

唯一索引

CREATE UNIQUE NONCLUSTERED INDEX [IX_Folders_UserParentFolder] ON [dbo].[Folders]
(
    [UserId] ASC,
    [ParentFolderId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

如何在

[Id]

和

[UserId]

上创建超级键/候选键，以便添加自引用外键？请记住，将主键从单个整数转换为复合键将中断引用文件夹表的其他表上的外键。这些外键不需要知道UserId

编辑：

根据Dan Guzman的建议/评论，添加数据库约束起到了作用。以下是约束的定义

约束定义

CREATE TABLE [dbo].[Folders](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [UserId] [int] NOT NULL,
    [ParentFolderId] [int] NULL
PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO

ALTER TABLE [dbo].[Folders]
ADD CONSTRAINT AK_Folders_UserId UNIQUE ([UserId], [Id])
GO

---

外键列引用的列必须是主键、唯一约束或唯一索引。我在此提出一个独特的约束条件：

ALTER TABLE dbo.Folders
ADD CONSTRAINT UQ_Folders_UserId_Id UNIQUE(UserId, Id);

---

在引用的列、UserId和Id上创建一个

唯一的
约束（或索引）。谢谢Dan，添加一个答案，我将奖励分数。。。