Sum of multiple queries to optimise number of queries performed - sql-server

Question

I have a query which must sum the values from several tables and add the result. The system is simply an inventory system and I'm trying to get the stock level by calculating incomings (deliveries), outgoings (issues) and adjustments to items.
As the stock level is a calculated value (sum(deliveries) - sum(issues)) + sum(adjustments) I am trying to create a function that will get this value with a minimal number of queries.
At current I have linq that performs three separate queries to get each summed value and then perform the addition/subtraction in my function, however I am convinced there must be a better way to calculate the value without having to do three separate queries.
The current function is as follows:
public static int GetStockLevel(int itemId)
{
using (var db = EntityModel.Create())
{
var issueItemStock = db.IssueItems.Where(x => x.ItemId == itemId).Sum(x => x.QuantityFulfilled);
var deliveryItemStock = db.DeliveryItems.Where(x => x.ItemId == itemId).Sum(x => x.Quantity);
var adjustmentsStock = db.Adjustments.Where(x => x.ItemId == itemId).Sum(x => x.Quantity);
return (deliveryItemStock - issueItemStock) + adjustmentsStock;
}
}
In my mind the SQL query is quite simple, so I have considered a stored procedure, however I think there must be a way to do this with linq.
Many thanks
Edit: Answer
Taking the code from Ocelot20's answer, with a slight change. Each of the lets can return a null, and if it does then linq throws an exception. Using the DefaultIfEmpty command will negate this, and return a 0 for the final calculation. The actual code I have used is as follows:
from ii in db.Items
let issueItems = db.IssueItems.Where(x => x.ItemId == itemId).Select(t => t.QuantityFulfilled).DefaultIfEmpty(0).Sum()
let deliveryItemStock = db.DeliveryItems.Where(x => x.ItemId == itemId).Select(t => t.Quantity).DefaultIfEmpty(0).Sum()
let adjustmentsStock = db.Adjustments.Where(x => x.ItemId == itemId).Select(t => t.Quantity).DefaultIfEmpty(0).Sum()
select (deliveryItemStock - issueItems) + adjustmentsStock);

Answer 1

Without knowing what your entities look like, you could do something like this:
public static int GetStockLevel(int itemId)
{
using (var db = EntityModel.Create())
{
// Note: Won't work if there are no IssueItems found.
return (from ii in db.IssueItems
let issueItems = db.IssueItems.Where(x => x.ItemId == itemId)
.Sum(x => x.QuantityFulfilled)
let deliveryItemStock = db.DeliveryItems.Where(x => x.ItemId == itemId)
.Sum(x => x.Quantity)
let adjustmentsStock = db.Adjustments.Where(x => x.ItemId == itemId)
.Sum(x => x.Quantity)
select issueItems + deliveryItemStock + adjustmentsStock).FirstOrDefault() ?? 0;
}
}
I tested a similar query on my own db and it worked in a single query. I suspect that since they all have a common ItemId, that using entity relations could make this look something like:
// Ideal solution:
(from i in db.Items
where i.Id == itemId
let issueItems = i.IssueItems.Sum(x => x.QuantityFulfilled)
let deliveryItemStock = i.DeliveryItems.Sum(x => x.Quantity)
let adjustmentsStock = i.Adjustments.Sum(x => x.Quantity)
select issueItems + deliveryItemStock + adjustmentsStock).SingleOrDefault() ?? 0;

Answer 2

Have you considered adding a view to the database that performs the calculations that you can then just use a simple select query (or SP) to return the values that you need?

Answer 3

I reckon this should work and the SQL generated is not particularly complex. If you think there is something wrong with it let me know and I will update my answer.
public static int GetStockLevel(int itemId)
{
using (var db = EntityModel.Create())
{
return db.IssueItems.Where(x => x.ItemId == itemId).GroupBy(x => x.ItemId)
.GroupJoin(db.DeliveryItems, x => x.First().ItemId, y => y.ItemId, (x, y) => new
{ Issues = x, Deliveries = y})
.GroupJoin(db.Adjustments, x=> x.Issues.First().ItemId, y=> y.ItemId, (x, y) => new
{
IssuesSum = x.Issues.Sum(i => i.QuantityFullfilled),
DeliveriesSum = x.Deliveries.Sum(d => d.Quantity),
AdjustmentsSum = y.Sum(a => a.Quantity)})
.Select(x => x.IssuesSum - x.DeliverysSum + x.AdjustmentsSum);
}
}