为什么INSERT的SQL数据库触发器实际上会更新未指定的字段?

SQL Server数据库表有以下4列:

CreatedBy
UpdatedBy
CreateDatetime
UpdateDatetime

我创建了一个用于更新的触发器 CreatedBy & CreateDatetime 列后 INSERT .

以下是该触发器的代码:

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TRIGGER [dbo].[TestDb_create_time]
ON [dbo].[trgn]
AFTER INSERT
AS 
BEGIN
    SET NOCOUNT ON;

    UPDATE TestDb 
    SET CreateDatetime = GETDATE(), CreatedBy = USER_NAME()
    FROM inserted
    WHERE inserted.TestDbID = TestDb.TestDbID
END

当触发器在insert之后运行时,除了更新 CreatedBy & CreateDatetime 专栏,它也更新了 UpdatedBy 和 UpdateDatetime .触发器中不包括最后两列。

为什么触发器也会更新它们?如何防止触发器更新未指定的列?

为这个问题添加更多细节:

SQL Server表:

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TABLE [dbo].[TestDb]
(
    [TestDbID] [int] IDENTITY(1,1) NOT NULL,
    [Item1] [varchar](30) NULL,
    [Item2] [varchar](30) NULL,
    [CreatedBy] [varchar](30) NULL,
    [UpdatedBy] [varchar](30) NULL,
    [CreateDatetime] [datetime] NULL,
    [UpdateDatetime] [datetime] NULL,

    CONSTRAINT [PK_TestDb] 
        PRIMARY KEY CLUSTERED ([TestDbID] ASC)
                WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
                      IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, 
                      ALLOW_PAGE_LOCKS = ON, OPTIMIZE_FOR_SEQUENTIAL_KEY = OFF) ON [PRIMARY]
) ON [PRIMARY]

的SQL Server触发器 插入 :

SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE TRIGGER [dbo].[TestDb_create_time]
ON [dbo].[TestDb]
AFTER INSERT
AS 
BEGIN
    SET NOCOUNT ON;

    UPDATE TestDb 
    SET CreateDatetime = GETDATE(), CreatedBy = USER_NAME()
    FROM inserted
    WHERE inserted.TestDbID = TestDb.TestDbID
END

ALTER TABLE [dbo].[TestDb] ENABLE TRIGGER [TestDb_create_time]

为了测试触发器,我使用两个不同的用户帐户执行了两条SQL语句:

INSERT INTO TestDb (item1) 
VALUES ('new record1')    --- connected with Windows account

INSERT INTO TestDb (item1) 
VALUES ('record2') --- connected with SQL user account

在插入之后 TestDb 表明 更新时间 列在2条新插入的记录中有值。这个 更新时间 触发器中不包括列。

为什么触发器会更新那个专栏?