Using a trigger to lock certain rows based on a condition - tsql

Question

Say for example, one has a table with certain rows, how does one prevent updates to certain rows based on a condition equalling true but allow the update to commit on all rows where the condition is false.
take this example where I "lock" all rows prior to 1/4/2007 by aborting the whole transaction
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
SET ANSI_PADDING ON
GO
CREATE TABLE [dbo].[tbl_TriggerTest] (
[ID] [int] IDENTITY(1,1) NOT NULL,
[Value] [varchar](25) NULL,
[Date] [datetime] NULL,
CONSTRAINT [PK_tbl_TriggerTest] 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
SET ANSI_PADDING OFF
GO
SET IDENTITY_INSERT [dbo].[tbl_TriggerTest] ON
GO
INSERT [dbo].[tbl_TriggerTest] ([ID], [Value], [Date]) VALUES (1, N'12', CAST(0x0000979A00000000 AS DateTime))
GO
INSERT [dbo].[tbl_TriggerTest] ([ID], [Value], [Date]) VALUES (2, N'13', CAST(0x00009A7500000000 AS DateTime))
GO
INSERT [dbo].[tbl_TriggerTest] ([ID], [Value], [Date]) VALUES (3, N'14', CAST(0x00009BE200000000 AS DateTime))
GO
INSERT [dbo].[tbl_TriggerTest] ([ID], [Value], [Date]) VALUES (4, N'4', CAST(0x00009D4F00000000 AS DateTime))
GO
SET IDENTITY_INSERT [dbo].[tbl_TriggerTest] OFF
GO
CREATE TRIGGER [dbo].[LockOldWelshRows] ON [dbo].[tbl_TriggerTest]
FOR UPDATE
AS
BEGIN
DECLARE #Count INT
SELECT #Count = COUNT([ID])
FROM INSERTED
WHERE [Date] < CONVERT(DATETIME, '01/04/2007 00:00:00', 103)
IF #Count > 0
BEGIN
RAISERROR('Rows prior to 01/04/2007 are locked',16,1)
ROLLBACK TRANSACTION
RETURN ;
END
END
GO
If one was to run the following:
UPDATE [tbl_TriggerTest] SET [Value] = [Value] + 'M'
The transaction would fail with error:
Msg 50000, Level 16, State 1, Procedure LockOldWelshRows, Line 12
Rows prior to 01/04/2007 are locked
Msg 3609, Level 16, State 1, Line 1
The transaction ended in the trigger. The batch has been aborted.
Is there a way of modifying this trigger to allow the transaction to commit, but only rows where the date is > 1/4/2007
This is a VERY brief example (the tables i am working with are far more complex), and if i am honest, i think it's cleaner if the whole transaction fails, i was just curious as to how it could be done.

Answer 1

You'd need a BEFORE aka INSTEAD OF trigger and filter to allowed rows only.
Untested:
CREATE TRIGGER [dbo].[LockOldWelshRows] ON [dbo].[tbl_TriggerTest]
INSTEAD OF UPDATE
AS
BEGIN
UPDATE
T
SET
T.SomeCol = I.SomeCol...
FROM
[dbo].[tbl_TriggerTest] T
JOIN
INSERTED I ON T.keycol = I.keycol
WHERE
T.[Date] >= '20070401';
END
GO