How to prevent insert when condition is met - sql

Question

I have a two tables
CREATE TABLE table1 (
ID int not null,
Status int not null,
TimeStamp datetime2(3)
CONSTRAINT pkId PRIMARY KEY (ID)
);
and
CREATE TABLE Status (
StatusID int not null,
StatusName varchar(64) not null
CONSTRAINT pStatus PRIMARY KEY (StatusID)
);
with inserted values
INSERT INTO table1 (ID,StatusID,TimeStamp) VALUES
(1,1,'2021-06-15 07:30:31'),
(2,2,'2021-07-15 07:30:31'),
(3,3,'2021-08-15 07:30:31'),
(4,4,'2021-09-15 08:30:31'),
(5,5,'2021-09-15 07:30:31'),
(6,5,'2021-09-15 07:30:31'),
(7,4,'2021-09-15 07:30:31'),
(8,2,'2021-09-15 07:30:31'),
(9,1,'2021-09-15 07:30:31');
and
INSERT INTO dbo.Status (StatusID,StatusName) VALUES
(1,'wants to enroll'),
(2,'granted enrollment'),
(3,'declined enrollment'),
(4,'attending the course'),
(5,'finished the course');
I wrote a trigger but it prevent all insert when the StatusID < 4 and what I want is to prevent insert into TimeStamp only when the condition is StatusID < 4. I use SQL Server.
CREATE TRIGGER dbo.Prevent_Insert4
ON dbo.table1
FOR INSERT
AS
SET NOCOUNT ON
IF EXISTS(SELECT *
FROM dbo.table1 t
JOIN inserted i ON t.ID=i.ID
WHERE t.StatusID<4)
BEGIN
RAISERROR('You can not update when StatusId value < 4', 16, 1)
ROLLBACK TRAN
SET NOCOUNT OFF
RETURN
END
SET NOCOUNT OFF
Thank you in advance.

Answer 1

Sounds like you just need a CHECK constraint
ALTER TABLE table1
ADD CONSTRAINT CHK_Status
CHECK(Status >= 4 OR TimeStamp IS NULL);
If you really, really wanted to use triggers for this (not recommended) then it should look like this
CREATE OR ALTER TRIGGER dbo.Prevent_Insert4
ON dbo.table1
FOR INSERT, UPDATE
AS
SET NOCOUNT ON;
IF EXISTS(SELECT 1
FROM inserted i
WHERE i.StatusID < 4
AND i.TimeStamp IS NOT NULL)
BEGIN
THROW 50001, 'You can not set TimeStamp when StatusId value < 4', 1;
-- no need for rollback, will happen automatically if you use THROW
END;
go

Answer 2

With your code you are doing rollback of inserted row when condition is met. If I understood you correctly you just to want override timestamp with null value if StatusId < 4.
If so, you should write that instead of rollback
IF EXISTS(SELECT *
FROM dbo.table1 t
JOIN inserted i ON t.ID=i.ID
WHERE t.StatusID<4)
BEGIN
UPDATE t SET t.TimeStamp = NULL
FROM inserted i
JOIN dbo.table1 t ON i.ID = t.ID
WHERE t.StatusID<4
END