Question

i need help to understand linq join. i found out a few topics related with this issue but i didnt found one with a good explanation of steps.

i normal query, i do this.

var q = from c in context.tableA
        select c;
        List<tableA> tableAList = q.ToList();

in q.ToList() its get executed the query, right?

here found some examples but i want to be clear about this,

 using (AdventureWorksEntities context = new AdventureWorksEntities())
 {
 ObjectSet<SalesOrderHeader> orders = context.SalesOrderHeaders;
 ObjectSet<SalesOrderDetail> details = context.SalesOrderDetails;

var query =
    from order in orders
    join detail in details
    on order.SalesOrderID equals detail.SalesOrderID
    where order.OnlineOrderFlag == true
    && order.OrderDate.Month == 8
    select new
    {
        SalesOrderID = order.SalesOrderID,
        SalesOrderDetailID = detail.SalesOrderDetailID,
        OrderDate = order.OrderDate,
        ProductID = detail.ProductID
    };

foreach (var order in query)
{
    Console.WriteLine("{0}\t{1}\t{2:d}\t{3}",
        order.SalesOrderID,
        order.SalesOrderDetailID,
        order.OrderDate,
        order.ProductID);
}
}

so from this example i can see that query can have more than 1 objetc but what about this "select new" ? is it called for each record in the DB ?

what type is that object? anyone i want or is order because is the first table in the query?

in case of the object is the first table, what happens if i need to have data that is not defined in this type of, i mean new attr.

other question, when is the query executed?

also, is this method good for response speed or are better solutions?

thx in advance. If there is a thread with this answers plz point me well.

Answer 1

in q.ToList() its get executed the query, right?

yes

what about this “select new” ? is it called for each record in the DB
?

new is just a new anonymous object, the query is ran against your table normally.

what type is that object? anyone i want or is order because is the
first table in the query?

it’s anonymous, you could do select new Order { though, if you have an Order class defined.

in case of the object is the first table, what happens if i need to
have data that is not defined in this type of, i mean new attr.

You would have to select it or add the attribute/property to your object/class.

other question, when is the query executed?

In the foreach loop

also, is this method good for response speed or are better solutions?

Yes, it’s fine