Merge inserted and deleted tables in SQL trigger I have a table with a Balance column of type decimal(18,0).

I want to get updates to its Balance field and reflect the changes in the parents of the updated row.

Currently I can easily subtract parent values by selecting from deleted table and add to parent values by selecting from inserted table in two different queries. Since the updates will trigger an other update and so the concept of recursive triggers (enabled in my database) comes to scene, I want to have the minimum number of updates.

Is the following the best way to merge inserted and deleted table and to get the real difference in Balance field?

Update MyTable
Set Balance += updated.Price
From (  
    Select  IsNull(inserted.ParentID, deleted.ParentID) As ParentID, 
            (IsNUll(inserted.Balance,0) - IsNull(deleted.Balance,0))  As Price 
            From inserted
            Full join deleted
            on inserted.ID = deleted.iD
            WHERE IsNull(inserted.ParentID, deleted.ParentID) IS NOT NULL
) As updated
INNER JOIN MyTable
ON MyTable.ID = updated.ParentID
Where updated.ParentID IS NOT NULL


For example changing Balance from 1000 to 800 should yield to -200 change in parent Balance fields.

Also, my code fires the trigger one extra time, so that for an update in level 3 I get 4 updates (trigger-firing) instead of 3.

This is on SQL Server 2012.
For me it is not exactly clear what you are trying to achieve, but it looks like propagating a balance change up into some tree (some chart of accounts, maybe?).

First, using triggers like this is very convoluted and error prone. Max recursion level for nested triggers is 32 and it is very hard to trace what it is going on. 

Maybe the following approach might be useful for you: create a temporary table based on your data to update (you may have multiple updates at once)

ID
ParentID
LevelNo -- should contain the level in the tree
OriginalBalance
NewBalance


Data can be populated using a recursive CTE, if update is touching only one node and you want to operate on that branch only.

For each level = LevelNo to 1
   Update parent with balance difference

This suggestion works only if your structure is a tree. If you provide us more details (table structure etc), we may come with a more concrete solution.