I just started learning LINQ to SQL, and so far I'm impressed with the easy of use and good performance.

I used to think that when doing LINQ queries like

from Customer in DB.Customers where Customer.Age > 30 select Customer

LINQ gets all customers from the database ("SELECT * FROM Customers"), moves them to the Customers array and then makes a search in that Array using .NET methods. This is very inefficient, what if there are hundreds of thousands of customers in the database? Making such big SELECT queries would kill the web application.

Now after experiencing how actually fast LINQ to SQL is, I start to suspect that when doing that query I just wrote, LINQ somehow converts it to a SQL Query string

SELECT * FROM Customers WHERE Age > 30

And only when necessary it will run the query.

So my question is: am I right? And when is the query actually run?

The reason why I'm asking is not only because I want to understand how it works in order to build good optimized applications, but because I came across the following problem.

I have 2 tables, one of them is Books, the other has information on how many books were sold on certain days. My goal is to select books that had at least 50 sales/day in past 10 days. It's done with this simple query:

from Book in DB.Books where (from Sale in DB.Sales where Sale.SalesAmount >= 50 && Sale.DateOfSale >= DateTime.Now.AddDays(-10) select Sale.BookID).Contains(Book.ID) select Book

The point is, I have to use the checking part in several queries and I decided to create an array with IDs of all popular books:

var popularBooksIDs = from Sale in DB.Sales where Sale.SalesAmount >= 50 && Sale.DateOfSale >= DateTime.Now.AddDays(-10) select Sale.BookID;

BUT when I try to do the query now:

from Book in DB.Books where popularBooksIDs.Contains(Book.ID) select Book

It doesn't work! That's why I think that we can't use thins kinds of shortcuts in LINQ to SQL queries, like we can't use them in real SQL. We have to create straightforward queries, am I right?