SQL - Set column value to the SUM of all references - sql

Question

I want to have the column "CurrentCapacity" to be the SUM of all references specific column.
Lets say there are three rows in SecTable which all have FirstTableID = 1. Size values are 1, 1 and 3.
The row in FirstTable which have ID = 1 should now have a value of 5 in the CurrentCapacity column.
How can I make this and how to do automatically on insert, update and delete?
Thanks!
FirstTable
+----+-------------+-------------------------+
| ID | MaxCapacity | CurrentCapacity |
+----+-------------+-------------------------+
| 1 | 5 | 0 (desired result = 5) |
+----+-------------+-------------------------+
| 2 | 5 | 0 |
+----+-------------+-------------------------+
| 3 | 5 | 0 |
+----+-------------+-------------------------+
SecTable
+----+-------------------+------+
| ID | FirstTableID (FK) | Size |
+----+-------------------+------+
| 1 | 1 | 2 |
+----+-------------------+------+
| 2 | 1 | 3 |
+----+-------------------+------+

Answer 1

In general, a view is a better solution than trying to keep a calculated column up-to-date. For your example, you could use this:
CREATE VIEW capacity AS
SELECT f.ID, f.MaxCapacity, COALESCE(SUM(s.Size), 0) AS CurrentCapacity
FROM FirstTable f
LEFT JOIN SecTable s ON s.FirstTableID = f.ID
GROUP BY f.ID, f.MaxCapacity
Then you can simply
SELECT *
FROM capacity
to get the results you desire. For your sample data:
ID MaxCapacity CurrentCapacity
1 5 5
2 5 0
3 5 0
Demo on SQLFiddle

Answer 2

Got this question to work with this trigger:
CREATE TRIGGER UpdateCurrentCapacity
ON SecTable
AFTER INSERT, UPDATE, DELETE
AS
BEGIN
SET NOCOUNT ON
DECLARE #Iteration INT
SET #Iteration = 1
WHILE #Iteration <= 100
BEGIN
UPDATE FirstTable SET FirstTable.CurrentCapacity = (SELECT COALESCE(SUM(SecTable.Size),0) FROM SecTable WHERE FirstTableID = #Iteration) WHERE ID = #Iteration;
SET #Iteration = #Iteration + 1
END
END
GO

Answer 3

Personally, I would not use a trigger either or store CurrentCapacity as a value since it breaks Normalization rules for database design. You have a relation and can already get the results by creating a view or setting CurrentCapacity to a calculated column.
Your view can look like this:
SELECT Id, MaxCapacity, ISNULL(O.SumSize,0) AS CurrentCapacity
FROM dbo.FirstTable FT
OUTER APPLY
(
SELECT ST.FirstTableId, SUM(ST.Size) as SumSize FROM SecTable ST
WHERE ST.FirstTableId = FT.Id
GROUP BY ST.FirstTableId
) O
Sure, you could fire a proc every time a row is updated/inserted or deleted in the second table and recalculate the column, but you might as well calculate it on the fly. If it's not required to have the column accurate, you can have a job update the values every X hours. You could combine this with your view to have both a "live" and "cached" version of the capacity data.