仅当使用了列的列表，并且 IDENTITY_INSERT 为 ON 时，才能在表 'Forms' 中为标识列指定显式值。

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if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[Category]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [dbo].[Category]
GO

if exists (select * from dbo.sysobjects where id = object_id(N'[dbo].[Forms]') and OBJECTPROPERTY(id, N'IsUserTable') = 1)
drop table [dbo].[Forms]
GO

CREATE TABLE [dbo].[Category] (
    [CategoryID] [int] IDENTITY (1, 1) NOT NULL ,
    [CategoryName] [varchar] (20) COLLATE Chinese_PRC_CI_AS NULL ,
    [FatherID] [int] NULL
) ON [PRIMARY]
GO

CREATE TABLE [dbo].[Forms] (
    [FormID] [int] IDENTITY (1, 1) NOT NULL ,
    [FormName] [nvarchar] (50) COLLATE Chinese_PRC_CI_AS NULL ,
    [FormUrl] [nvarchar] (50) COLLATE Chinese_PRC_CI_AS NULL ,
    [Form_category_id] [int] NULL ,
    [target] [char] (10) COLLATE Chinese_PRC_CI_AS NULL
) ON [PRIMARY]
GO
CREATE FUNCTION IsLeaf (@cat_id int) 
RETURNS int AS 
BEGIN

declare @count int
select @count = (select count(*) from Category where FatherID=@cat_id)
if (@count=0)
return 1
return 0

END


之后插入数据的时候提示那个错误
解决:
1\表中存在标识列的时候,对其执行插入操作,要指明具体的列.  
  因为标识列ID是不允许写的.  
2\你按提示做不就行了，要不就不要插入自增字段的值