How to prevent insert when condition is met

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

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

Answer 2

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