How to use sqlite db functions to set values from calculation - sql

Question

I have an app (its ruby on rails but it shouldn't matter) where I am bulk loading items and quantities per branch.
The database is sqlite.
Say I have a two tables: items and quantities.
items columns:
id
quantity
quantities columns:
id
item_id
branch_id
value
I need to be able to loop through all records in the items table set the item.quantity to the sum of all quantity.value where item_id matches item.id
I would like to know if there is a way to do this all within a sql query (the select and update) so that I don't have to pull the data out of the database, do the calculation and then write the updated quantity back to each item, instead I would like to have it all occur in the database using sql statements and functions.
Is this possible in a sql query function db side?
example:
item[1] = {id=>1,quantity=>0}
quantity[1] = {id=>1,item_id=>1,value=>100,branch_id=>1}
quantity[2] = {id=>2,item_id=>1,value=>200,branch_id=>2}
quantity[3] = {id=>3,item_id=>1,value=>300,branch_id=>3}
item[1].quantity = quantity[1].value + quantity[2].value + quantity[3].value

Answer 1

You can do this with an update query. In this case, you can use a correlated subquery to calculate the sum of the quantities for a given item:
update item
set quantity = (select sum(value) from quantities where item.id = quantities.item_id);
EDIT:
If you want to speed it up putting an index on quantities(item_id, value).
You could also summarize quantities in a temporary table and use this instead. The index, however, however, should be sufficient for performance.

Answer 2

You are duplicating your data! Which means you must always keeping everything in sync.
I would prefer to create a view and get total quantities from there:
CREATE TABLE items(id, ...);
CREATE TABLE quantities(id, item_id, branch_id, value, ...);
CREATE VIEW items_sum AS SELECT items.*, SUM(quantities.value) AS quantity FROM items LEFT JOIN quantities ON items.id=quantities.item_id GROUP BY items.id;
As items_sum is a query, you don't have to update it. SELECT * FROM items_sum;will return desired results for total quantities.