Question

In my database, I have a few columns whose changes are to be tracked. Let us have A, B and C columns. User and administrator can change the values in these columns. If a cell value(column, row) is changed by administrator, user should not able to change it.

To solve this, I am thinking to maintain a log table for cell changes with fields – Row ID, Column ID, User Changed.

Note : I am using Sql Server 2008, C#.

Please comment on my idea. thanks.

Answer 1

SQL Server does not maintain information about who changed a column last. Therefore you would need to track this information yourself using an INSTEAD OF or AFTER trigger. You would also need to decide what you want to do if a user tries to update Column A and Column B, but the administrator has updated Column A only – do you want to allow the user to update Column B, or should their whole update fail?

You could have an additional column in the table that indicates ChangedByAdmin for each column:

ALTER TABLE dbo.MyTable ADD A_ChangedByAdmin BIT NOT NULL DEFAULT 0;
ALTER TABLE dbo.MyTable ADD B_ChangedByAdmin BIT NOT NULL DEFAULT 0;
ALTER TABLE dbo.MyTable ADD C_ChangedByAdmin BIT NOT NULL DEFAULT 0;

(Using a separate table seems overkill unless you want to maintain the history.)

Now you could have an INSTEAD OF trigger that follows your rules. Just a rough idea off the cuff:

CREATE TRIGGER dbo.Update_MyTable
FOR dbo.MyTable
INSTEAD OF UPDATE
AS
BEGIN
  SET NOCOUNT ON;

  DECLARE @a BIT = CASE USER_NAME() WHEN 'Administrator' THEN 1 ELSE 0 END;

  IF UPDATE(A)
  BEGIN
    UPDATE t
      SET A = i.A, A_ChangedByAdmin = @a
      FROM dbo.MyTable AS t
      INNER JOIN inserted AS i
      ON t.key = i.key
      INNER JOIN deleted AS d
      ON i.key = d.key
      AND i.A <> d.A -- why bother if the value is the same
      WHERE (@a = 1 OR d.A_ChangedByAdmin = 0);
  END
  -- repeat for B and C

  -- then update the unrestricted columns:
  UPDATE t SET D = i.D, E = i.E --, etc.
    FROM dbo.MyTable AS t
    INNER JOIN inserted AS i
    ON t.key = i.key;
END
GO