仅当列值更改时，T-SQL触发器

Question

I'm using SQL Server 2008 and have the following trigger:

USE [gatordata]
GO
/****** Object:  Trigger [dbo].[TriggerUserAssigned] Script Date:2/20/2016 3:42:05 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER TRIGGER [dbo].[TriggerUserAssigned]
ON [dbo].[User]
AFTER UPDATE
AS
IF UPDATE([AssignedToUser]) 
BEGIN
declare @Id int;
declare @AssignedToUser nvarchar(50);
declare @TimeStamp datetime;
declare @Hour bit;
declare @RushService bit;
declare @Status nvarchar(50);
select @Id=i.Id from inserted i;
select @AssignedToUser=i.AssignedToUser from inserted i;
select @Hour=i.[Hour] from inserted i;
select @RushService=i.[RushService] from inserted i;
select @Status=i.[Status] from inserted i;

INSERT INTO [Messages]  
(RequestID,AssignedToUser,[TimeStamp],[Hour],RushService,[Status],MessageText) VALUES (@Id,@AssignedToUser,GETDATE(),@Hour,@RushService,@Status,'New Critical Request')
END

This is working fine except I need to modify it so it only triggers the insert to the "Messages" table when the column "AssignedToUser" value actually changes to a new value. If the value of "AssignedToUser" remains the same when the row is updated the trigger should not fire. Thanks in advance SO friends...

Answer 1

Assuming that you have a primary key on Users , this is not difficult. Let me assume that primary key is RequestId .

Then:

insert into messages( . . . )
    select i.id, i.assignedtouser, i.hour, i.rushservice, i.status
    from inserted i join
         deleted d
         on i.requestid = d.requestid
    where i.assignedtouser <> d.assignedtouser;

Note: You should never assume that inserted and deleted have only one row, as your original code does. This will simply fail when someone attempts to update multiple rows.

You should also explicitly include the column list in an insert statement. So the . . . . . . is for the list of columns in the messages table.

Answer 2

I think this should do it :

USE [gatordata]
GO
/****** Object:  Trigger [dbo].[TriggerUserAssigned] Script Date:2/20/2016 3:42:05 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER TRIGGER [dbo].[TriggerUserAssigned]
ON [dbo].[User]
AFTER UPDATE
AS
    INSERT  [Messages](
            [RequestID],        [AssignedToUser],   [TimeStamp],            [Hour],
            [RushService],      [Status],           [MessageText]) 
    SELECT 
            i.[Id],             i.[AssignedToUser], GETDATE(),              i.[Hour],
            i.[RushService],    i.[Status],         'New Critical Request'
    FROM    [inserted]  i
    JOIN    [deleted]   d       ON d.[Id] = i.[Id] 
    WHERE   d.[AssignedToUser] <> i.[AssignedToUser];
GO

By joining the inserted table to the deleted table, you should be able to compare the values in that column to determine whether it has changed. The added benefit is you can select straight from the inserted table instead of having to assign everything to variables.