Sql Trigger to add new row when table update - sql

Question

I am new to sql triggers and learning as developing triggers for asp.net application.i am having case where I need to first save table and then edit the same table - this edit create new row in different table but the the problem is every time I edit the table it create new row in different table I want to create row in different table for only first edit.
Dividing my problem for readability.
I have two tables:
Table A and table B
I have written trigger on table A that add row in table B.
Problem:
Every time I edit row in table A a new row get added to table B. (So every edit create new row)
Required result:
I want my trigger to add ONLY one row in table B for the first edit in table A but not for subsequent edits.
I am using update triggers.
Any example with code would be great
Thanks you much in advance .
Create TRIGGER [dbo].[triggerName] ON [dbo].[databaseName]
For Update
As
Begin
DECLARE #i int
DECLARE #d int
DECLARE #action char(6)
DECLARE #Car VARCHAR(20)
IF (##ROWCOUNT = 0) RETURN
SELECT #i = Count(*) From Inserted
SELECT #d = Count(*) From Deleted
SELECT #action = CASE
WHEN (#i <> 0) and (#d <> 0) THEN 'UPDATE'
WHEN (#i = 0) and (#d <> 0) THEN 'DELETE'
WHEN (#i <> 0) and (#d = 0) THEN 'INSERT'
End
SELECT #Car = A From inserted
IF #action = 'UPDATE' AND #Car in ('BMW')
Begin
INSERT INTO Tableb (c,d,f)
Select c,d,f from inserted
End

Answer 1

Your trigger has some flaws in it.
First, You don't need to test if it was fired because of update, insert or delete. The trigger is specified for update, so inserts and deletes will not fire it anyway.
Second, SELECT #Car = A From inserted will raise an error whenever you update more then one row in the table.
Third, As you said, this will insert a record in tableB for every update, while you want it to insert a record only for the first update done (I assume one for the first update on any row, so if you update row 1 then insert, update row 2 then another insert, and update row 1 again don't insert).
I would write it like this:
Create TRIGGER [dbo].[triggerName] ON [dbo].[tableName]
For Update
As
Begin
INSERT INTO Tableb (c,d,f)
Select c,d,f
from inserted i
left join Tableb t ON(i.c = t.c and i.d = t.d and i.f = t.f)
where t.id is null -- or some other non-nullable column
and i.a = 'BMW'
End

Answer 2

You can modify your SQL trigger to execute only after INSERT
CREATE TRIGGER dbo.myTable_Insert
ON dbo.myTable
AFTER INSERT
AS
It is possible to create SQL trigger to run after insert, update or delete as seen in the referred tutorial