Question

Imagine to have a table defined as

CREATE TABLE [dbo].[Price](
    [ID] [int] NOT NULL,
    [StartDate] [datetime] NOT NULL,
    [Price] [int] NOT NULL
)

where ID is the identifier of an action having a certain Price. This price can be updated if necessary by adding a new line with the same ID, different Price, and a more recent date.

So with a set of a data like

ID StartDate Price

1  01/01/2009 10
1  01/01/2010 20
2  01/01/2009 10
2  01/01/2010 20

How to obtain a set like the following?

1  01/01/2010 20
2  01/01/2010 20

Answer 1

In SQL, there are several ways to say it. Here’s one that uses a subquery:

SELECT *
  FROM Price p
 WHERE NOT EXISTS (
        SELECT *
          FROM Price
         WHERE ID = p.ID
           AND StartDate > p.StartDate
       )

This translates fairly trivially to LINQ:

var q = from p in ctx.Price
        where !(from pp in ctx.Price
                where pp.ID == p.ID
                   && pp.StartDate > p.StartDate
                select pp
               ).Any()
        select p;

Or should I say, I think it does. I’m not in front VS right now, so I can’t verify that this is correct, or that LINQ will be able to convert it to SQL.

Minor quibble: Don’t use the name ID to store a non-unique value (the type, in this case). It’s confusing.