Question

Here’s some LINQ to select all order details. It creates a join with the product table to get the product name:

var query = from od in db.Order_Details
            join p in db.Products on od.ProductID equals p.ProductID
            select new { od.OrderID, od.ProductID, p.ProductName };

Here’s how I would do it if I didn’t know that Join existed:

var query = from od in db.Order_Details
            select new { od.OrderID, 
                         od.ProductID, 
                         ProductName = (from p in db.Products 
                                        where p.ProductID == od.ProductID 
                                        select p.ProductName).First()
            };

They generate different underlying SQL code. Is the first method faster than the second, and if so why?

ETA:

db.Log for join query:

SELECT [t0].[OrderID], [t0].[ProductID], [t1].[ProductName]
FROM [dbo].[Order Details] AS [t0]
INNER JOIN [dbo].[Products] AS [t1] ON [t0].[ProductID] = [t1].[ProductID]

db.Log for 2nd query:

SELECT [t0].[OrderID], [t0].[ProductID], (
    SELECT TOP (1) [t1].[ProductName]
    FROM [dbo].[Products] AS [t1]
    WHERE [t1].[ProductID] = [t0].[ProductID]
    ) AS [ProductName]
FROM [dbo].[Order Details] AS [t0]

Answer 1

Joins are typically faster than equivalent nested selects since DBMS’ are very good at optimizing joins, although a good SQL compiler might optimize them to the same SQL anyway. You should use the one which makes your purpose more clear, which is probably the join in this case.