Database trigger to update a row on parent table after child table has been updated - sql

Question

First time creating a Database trigger. I have a child table that when its cost column is updated, I need its parent table to also update its cost column to reflect the change.
Here is my sorry attempt so far. That's obviously not working. I am having a problem figuring out how to extract the total cost as a variable and store it in the parent table.
My current approach assumes a static id vaule at the moment. I am not entirely sure how to dynamically determine the id value of the row that was updated.
CREATE TRIGGER ParentCost_Update
ON ChildTable
AFTER INSERT, UPDATE
AS
SELECT SUM(Cost) AS TotalCost FROM ChildTable where parent_id=2080
UPDATE ParentTable
SET Cost=TotalCost
where id=parent_id;
GO
This current script is returning an error
Msg 207, Level 16, State 1, Procedure ParentCost_Update, Line 9
Invalid column name 'TotalCost'.

Answer 1

You need to be careful in triggers, in that there could be more than one row updated. Therefore, you need to do row-based handling.
To obtain the newly inserted / updated row, use the inserted and deleted pseudo rows.
You will almost certainly also going to need to implement a deleted trigger as well, viz, if a row is removed from a child table that the parent will need to be recalculated.
Here's a row based take, using a CTE to map your two-step process above , as follows:
CREATE TRIGGER ParentCost_Update
ON ChildTable
AFTER INSERT, UPDATE, DELETE
AS
SET NOCOUNT ON;
WITH cteParentsAffected AS
(
SELECT ins.parent_id
FROM inserted ins
UNION
SELECT del.parent_id
FROM deleted del
)
, cteTotal AS
(
SELECT ct.parent_id, SUM(ct.Cost) AS TotalCost
FROM ChildTable ct
INNER JOIN cteParentsAffected par
ON ct.parent_id = par.parent_id
GROUP BY ct.parent_id
)
UPDATE pt
SET Cost=cte.TotalCost
FROM ParentTable pt
INNER JOIN cteTotal cte
ON id=cte.parent_id;
GO
With a SqlFiddle here

Answer 2

Try this
Update parenttable set total= (select sum(total) from childtable c where c.id= parent table.id)
Where id in (select id from inserted)
Change the table and column names.