LINQ nested joins - linq

Question

Im trying to convert a SQL join to LINQ. I need some help in getting the nested join working in LINQ.
This is my SQL query, Ive cut it short just to show the nested join in SQL:
select distinct
txtTaskStatus as TaskStatusDescription,
txtempfirstname+ ' ' + txtemplastname as RaisedByEmployeeName,
txtTaskPriorityDescription as TaskPriorityDescription,
dtmtaskcreated as itemDateTime,
dbo.tblTask.lngtaskid as TaskID,
dbo.tblTask.dtmtaskcreated as CreatedDateTime,
convert(varchar(512), dbo.tblTask.txttaskdescription) as ProblemStatement,
dbo.tblTask.lngtaskmessageid,
dbo.tblMessage.lngmessageid as MessageID,
case when isnull(dbo.tblMessage.txtmessagesubject,'') <> '' then txtmessagesubject else left(txtmessagedescription,50) end as MessageSubject,
dbo.tblMessage.txtmessagedescription as MessageDescription,
case when dbo.tblMessage.dtmmessagecreated is not null then dbo.tblMessage.dtmmessagecreated else CAST(FLOOR(CAST(dtmtaskcreated AS DECIMAL(12, 5))) AS DATETIME) end as MessageCreatedDateTime
FROM
dbo.tblAction RIGHT OUTER JOIN dbo.tblTask ON dbo.tblAction.lngactiontaskid = dbo.tblTask.lngtaskid
LEFT OUTER JOIN dbo.tblMessage ON dbo.tblTask.lngtaskmessageid = dbo.tblMessage.lngmessageid
LEFT OUTER JOIN dbo.tblTaskCommentRecipient
RIGHT OUTER JOIN dbo.tblTaskComment ON dbo.tblTaskCommentRecipient.lngTaskCommentID = dbo.tblTaskComment.lngTaskCommentID
ON dbo.tblTask.lngtaskid = dbo.tblTaskComment.lngTaskCommentTaskId

Answer 1

A more seasoned SQL programmer wouldn't join that way. They'd use strictly left joins for clarity (as there is a strictly left joining solution available).
I've unraveled these joins to produce a hierarchy:
Task
Action
Message
TaskComment
TaskCommentRecipient
With associations created in the linq to sql designer, you can reach these levels of the hierarchy:
//note: these aren't outer joins
from t in db.Tasks
let actions = t.Actions
let message = t.Messages
let comments = t.TaskComments
from c in comments
let recipients = c.TaskCommentRecipients
DefaultIfEmpty produces a default element when the collection is empty. Since these are database rows, a default element is a null row. That is the behavior of left join.
query =
(
from t in db.Tasks
from a in t.Actions.DefaultIfEmpty()
from m in t.Messages.DefaultIfEmpty()
from c in t.Comments.DefaultIfEmpty()
from r in c.Recipients.DefaultIfEmpty()
select new Result()
{
TaskStatus = ???
...
}
).Distinct();
Aside: calling Distinct after a bunch of joins is a crutch. #1 See if you can do without it. #2 If not, see if you can eliminate any bad data that causes you to have to call it. #3 If not, call Distinct in a smaller scope than the whole query.
Hope this helps.

Answer 2

SELECT [t0].[OrderID], [t0].[CustomerID], [t0].[EmployeeID], [t0].[OrderDate], [t0].[RequiredDate], [t0].[ShippedDate], [t0].[ShipVia], [t0].[Freight], [t0].[ShipName], [t0].[ShipAddress], [t0].[ShipCity], [t0].[ShipRegion], [t0].[ShipPostalCode], [t0].[ShipCountry]
FROM [Orders] AS [t0]
LEFT OUTER JOIN ([Order Details] AS [t1]
INNER JOIN [Products] AS [t2] ON [t1].[ProductID] = [t2].[ProductID]) ON [t0].[OrderID] = [t1].[OrderID]
can be write as
from o in Orders
join od in (
from od in OrderDetails join p in Products on od.ProductID equals p.ProductID select od)
on o.OrderID equals od.OrderID into ood from od in ood.DefaultIfEmpty()
select o