Create trigger to insert a column value into other column of same table SQL Server 2005

Question

How to create trigger to insert a value in a column same as value of another column of same table of a newly inserted row in the table.

Suppose I have table like below

ColumnA | ColumnB

I want columnB value to be inserted into ColumnA as soon as row gets inserted into table or columnB value gets updated . But it should not be vice versa ie insert columnA value into columnB

.Below code handles INSERT only , please help me how to handle both Insertion and Updation of table ie when columnB value gets inserted or updated.

CREATE TRIGGER inserupdate
   ON  triggertestTable
   AFTER INSERT,UPDATE
AS 
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    -- Insert statements for trigger here
    declare @value int

    select @value=columnB from inserted

    update triggertestTable
    set columnA=@value

END
GO

This works fine if values inserted like below

insert into triggertestTable(columnB) values('xyz')

ColumnB value gets inserted into columnA

ColumnA | ColumnB
  xyz   |  xyz 

But null value gets updated in both if some other application inserts value in columnA

 insert into triggertestTable(columnA) values('pqr')

Now records are

ColumnA | ColumnB
  xyz   |  xyz 
  NULL  |  NULL

The correct recordset should be like below

  ColumnA   | ColumnB
      xyz   |  xyz 
      pqr   |  NULL

How to solve this problem.

Answer 1

Try this trigger (it will copy the values from ColumnB to ColumnA when are inserted values into ColumnB or when are updated the values from ColumbB):

CREATE TRIGGER trgIU_triggertestTable_UpdateColumnAWhenColumnB
   ON  dbo.triggertestTable
   AFTER INSERT,UPDATE
AS 
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    IF UPDATE(ColumnB)
    BEGIN
        UPDATE  dbo.triggertestTable
        SET     ColumnA=i.ColumnB
        FROM    inserted i
        INNER JOIN dbo.triggertestTable t ON i.MyID=t.MyID
        LEFT JOIN deleted d ON i.MyID=d.MyID
        WHERE   d.MyID IS NULL AND i.ColumnB IS NOT NULL -- Row was inserted
        OR      d.MyID IS NOT NULL -- Row was updated
    END
END
GO

I used this table:

CREATE TABLE dbo.triggertestTable(
    MyID INT IDENTITY(1,1) PRIMARY KEY, -- MyID should be a PRIMARY KEY or a mandatory(NOT NULL) UNIQUE constraint
    ColumnA VARCHAR(100),
    ColumnB VARCHAR(100)
);
GO