Question

I have 2 SQL statements that basically do the same thing, that is, retrieve the last record from a table based on a datetime field for a group of records. I am using the data-first Entity Framework model. How would I write either of these SQL statements using LINQ Lambda functions?

ie,

var u = db.AccessCodeUsage.Where(...).GroupBy(...)

rather than

var u = from a in db.AccessCodeUsage
        where ...
        group by ...

SQL Statements:

SELECT  *
FROM    AccessCodeUsage a  
WHERE   NOT EXISTS (SELECT  1 
                    FROM    AccessCodeUsage 
                    WHERE   LocationId = a.LocationId
                    AND     Timestamp > a.Timestamp)

SELECT    a.*
FROM    AccessCodeUsage a
WHERE   a.Timestamp = 
        (SELECT MAX(Timestamp)
        FROM    AccessCodeUsage
        WHERE   a.LocationId = LocationId
        AND     a.AccessCode = AccessCode
        GROUP   By LocationId, AccessCode)

Answer 1

I ended up using the following which corresponds to the first SQL statement.

// Retrieve only the latest (greatest value in timestamp field) record for each Access Code
var last = AccessCodeUsages.Where(u1 => !AccessCodeUsages
                           .Any(u2 => u2.LocationId == u1.LocationId &&
                                      u2.AccessCode == u1.AccessCode &&
                                      u2.Timestamp > u1.Timestamp));