How to set a default value for one column in SQL based on another column

Question

I\'m working with an old SQL 2000 database and I don\'t have a whole lot of SQL experience under my belt. When a new row is added to one of my tables I need to assign a defa

Answer 1

So, for example, in a TAG table (where tags are applied to posts) if you want to count one tag as another...but default to counting new tags as themselves, you would have a trigger like this:

CREATE TRIGGER [dbo].[TR_Tag_Insert]
   ON  [dbo].[Tag]
   AFTER INSERT
AS 
BEGIN
   SET NOCOUNT ON;

   UPDATE dbo.Tag 
   SET [CountAs] = I.[ID]
   FROM INSERTED AS I
   WHERE I.[CountAs] IS NULL
   AND dbo.Tag.ID = I.ID
END

Answer 2

Use a trigger as suggested by Stephen Wrighton:

CREATE TRIGGER [myTable_TriggerName] ON dbo.myTable FOR INSERT        
AS
SET NOCOUNT ON
UPDATE myTable
SET
    timeValue = '2 hours' -- assuming string values
where ID in (
    select ID
    from INSERTED
    where
        timeValue = ''
        AND workCategory = 'A'
)

Answer 3

Yeah, trigger.

Naturally, instead of hard-coding the defaults, you'll look them up from a table.

Expanding on this, your new table then becomes the work_category table (id, name, default_hours), and you original table maintains a foreign key to it, transforming fom (id, work_category, hours) to (id, work_category_id, hours).

Answer 4

Generally I steer away from triggers. Almost all dbms have some sort of support for constraints.

I find them easier to understand , debug and maintain.

Answer 5

If what you are looking for is to define a column definition based on another column you can do something like this:

create table testable 
(
    c1 int, 
    c2 datetime default getdate(), 
    c3 as year(c2)
);

insert into testable (c1) select 1

select * from testable;

Your result set should look like this :

c1 | c2                      | c3
1  | 2013-04-03 17:18:43.897 | 2013

As you can see AS (in the column definition) does the trick ;) Hope it helped.

Answer 6

I would use a trigger on Insert.

Just check to see if a value has been assigned, and if not, go grab the correct one and use it.