当 IDENTITY_INSERT 设置为 OFF 时，不能向表 中的标识列插入显式值错误的解决方法

一个主键。两个外键，
把两个外键改为非空就行了。

CREATE TABLE [dbo].[User_Compare](
    [Id] [int] IDENTITY(1,1) NOT NULL,
    [UserId] [int] NOT NULL,
    [PatentId] [int] NOT NULL,
 CONSTRAINT [PK_User_Compare] 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].[User_Compare]  WITH CHECK ADD  CONSTRAINT [FK_User_Compare_Patents] FOREIGN KEY([PatentId])
REFERENCES [dbo].[Patents] ([Id])
GO

ALTER TABLE [dbo].[User_Compare] CHECK CONSTRAINT [FK_User_Compare_Patents]
GO

ALTER TABLE [dbo].[User_Compare]  WITH CHECK ADD  CONSTRAINT [FK_User_Compare_Users] FOREIGN KEY([UserId])
REFERENCES [dbo].[Users] ([Id])
GO

ALTER TABLE [dbo].[User_Compare] CHECK CONSTRAINT [FK_User_Compare_Users]
GO