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how can I implement CASE WHEN with LINQ?

I wrote a T-SQL code which has used case when in select scope. We couldn't use t-sql or store procedure in application, because of that I need to convert follong code to LINQ. Is there any way to change this code to linq quickly?
SELECT
T.TaskID,
SUM(CASE WHEN T.LogDate<#fromDate AND T.TaskStatusID=2 THEN ISNULL(DA_CHILD.Score,0)*(T.DoneScore/100) ELSE 0 END) PreAmount,
SUM(CASE WHEN T.LogDate>=#fromDate AND T.LogDate<=#toDate AND T.TaskStatusID=2 THEN ISNULL(DA_CHILD.Score,0)*(T.DoneScore/100) ELSE 0 END) CurAmount
FROM
NetTasks$ T
INNER JOIN NetDeviceActions DA ON DA.DeviceActionID=T.DeviceActionID
LEFT JOIN NetFinancialInfoDetail FID ON FID.TaskID=T.TaskID
INNER JOIN NetActionParents AP ON AP.ParentID=DA.ActionID
INNER JOIN NetDeviceActions DA_CHILD ON DA_CHILD.ActionID=AP.ChildID AND
DA_CHILD.DeviceID=DA.DeviceID AND
DA_CHILD.ContractInfoID=DA.ContractInfoID
WHERE
T.ParentTaskID = 0 AND
T.FinishDate<=#toDate AND
DA.ContractInfoID=9
GROUP BY
T.TaskID, T.DoneScore,T.FinishDate

In LINQ you can use C# statements so CASE WHEN is actually not hard.
Assuming you have finished all the joining into a query object called values, you can use something like below for the grouping and select:
var q = from a in values
group a by new {a.TaskID, a.DoneScore, a.FinishDate} into g
select new {
g.Key.TaskID,
PreAmount = g.Where(x => x.LogDate < fromDate && x.TaskStatusID == 2 && x.DA_CHILD.HasValue).Select(x => x.DoneScore).Sum(),
CurAmount = g.Where(x => x.LogDate >= fromDate && x.LogDate < toDate && x.TaskStatusID == 2 && x.DA_CHILD.HasValue).Select(x => x.DoneScore).Sum()
};
And of course, a friendly reminder, left joining in LINQ is very tedious.

Are you just looking for a simple where clause in your statement? (Though I admit this LINQ query is not going to be particularly simple.)
https://msdn.microsoft.com/en-us/library/bb397927.aspx
I advise building it up slowly.

With such good looking SQL, would you not be happier using QueryFirst and forgetting about Linq? You run your SQL directly in your C# app.
disclaimer : I wrote QueryFirst

Convert query from SQL to LINQ

I need to convert this SQL query to LINQ:
SELECT COUNT(result1.Id) AS total,
result1.OccurrenceDate,
result1.Path,
result1.Message,
result1.StackTrace
FROM (SELECT * FROM tbllog ORDER BY OccurrenceDate DESC) AS result1
GROUP BY result1.Path, result1.Message ORDER BY total DESC
But I'm not getting success. I've tried in so many ways, nothing works.
Some help?

Create a linq query out of the following:
SELECT * FROM tbllog ORDER BY OccurrenceDate DESC AS result1
GROUP BY result1.Path, result1.Message ORDER BY total DESC
Once that is done, write some linq to get your counts.

var results = _dbContext.tbllog
.GroupBy( t => new { t.Path, t.Message })
.Select( g => new
{
Total = g.Count(),
Path = g.Key.Path,
Message = g.Key.Message
}).OrderBy(p => p.Total);
It is worth to mention that your sql query is invalid because the properties OccurrenceDate and StackTrace is not in the GROUP BY clause nor with in an aggregate function, therefore it is invalid query. The same with the LINQ query. You should determine what you want to do with them. Either include them in the group by or use an aggregate function to select the appropriate value for each group.

I don't know what the context is for your question, but if you don't need to modify the query such that it executes with additional parameters, I would highly recommend executing the query as SQL. This is supported on all the ORMs that I am aware of (Entity Framework, NHiberate, LINQ to SQL, etc).
Erick

How to force LINQ to SQL to evaluate the whole query in the database?

I have a query which is fully translatable to SQL. For unknown reasons LINQ decides the last Select() to execute in .NET (not in the database), which causes to run a lot of additional SQL queries (per each item) against database.
Actually, I found a 'strange' way to force the full translation to SQL:
I have a query (this is a really simplified version, which still does not work as expected):
MainCategories.Select(e => new
{
PlacementId = e.CatalogPlacementId,
Translation = Translations.Select(t => new
{
Name = t.Name,
// ...
}).FirstOrDefault()
})
It will generates a lot of SQL queries:
SELECT [t0].[CatalogPlacementId] AS [PlacementId]
FROM [dbo].[MainCategories] AS [t0]
SELECT TOP (1) [t0].[Name]
FROM [dbo].[Translations] AS [t0]
SELECT TOP (1) [t0].[Name]
FROM [dbo].[Translations] AS [t0]
...
However, if I append another Select() which just copies all members:
.Select(e => new
{
PlacementId = e.PlacementId,
Translation = new
{
Name = e.Translation.Name,
// ...
}
})
It will compile it into a single SQL statement:
SELECT [t0].[CatalogPlacementId] AS [PlacementId], (
SELECT [t2].[Name]
FROM (
SELECT TOP (1) [t1].[Name]
FROM [dbo].[Translations] AS [t1]
) AS [t2]
) AS [Name]
FROM [dbo].[MainCategories] AS [t0]
Any clues why? How to force the LINQ to SQL to generate a single query more generically (without the second copying Select())?
NOTE: I've updated to query to make it really simple.
PS: Only, idea I get is to post-process/transform queries with similar patterns (to add the another Select()).

When you call SingleOrDefault in MyQuery, you are executing the query at that point which is loading the results into the client.
SingleOrDefault returns IEnumerable<T> which is no longer an IQueryable<T>. You have coerced it at this point which will do all further processing on the client - it can no longer perform SQL composition.

Not entirely sure what is going on, but I find the way you wrote this query pretty 'strange'. I would write it like this, and suspect this will work:
var q = from e in MainCategories
let t = Translations.Where(t => t.Name == "MainCategory"
&& t.RowKey == e.Id
&& t.Language.Code == "en-US").SingleOrDefault()
select new TranslatedEntity<Category>
{
Entity = e,
Translation = new TranslationDef
{
Language = t.Language.Code,
Name = t.Name,
Xml = t.Xml
}
};
I always try to separate the from part (selection of the datasources) from the select part (projection to your target type. I find it also easier to read/understand, and it generally also works better with most linq providers.

You can write the query as follows to get the desired result:
MainCategories.Select(e => new
{
PlacementId = e.CatalogPlacementId,
TranslationName = Translations.FirstOrDefault().Name,
})
As far as i'm aware, it's due to how LINQ projects the query. I think when it see's the nested Select, it will not project that into multiple sub-queries, as essentially that would be what would be needed, as IIRC you cannot use multiple return columns from a sub-query in SQL, so LINQ changes this to a query-per-row. FirstOrDefault with a column accessor seems to be a direct translation to what would happen in SQL and therefore LINQ-SQL knows it can write a sub-query.
The second Select must project the query similar to how I have written it above. It would be hard to confirm without digging into a reflector. Generally, if I need to select many columns, I would use a let statement like below:
from e in MainCategories
let translation = Translations.FirstOrDefault()
select new
{
PlacementId = e.CatalogPlacementId,
Translation = new {
translation.Name,
}
})

Sub Query in LINQ

I have an SQL Query as given below
SELECT ui.PageStyleCss
FROM UserImages ui
WHERE ui.UserImageId IN
( SELECT inv.UserImageId
FROM Invitation inv
JOIN InviteeEmails invEmails ON
inv.InviteID = invEmails.InviteID
WHERE invEmails.InviteGUID = #InviteGUID
)
How can I write this in LINQ?
Thanks

My wild guess is that you're using LINQ to SQL. It would be nice if you mentioned this, along with details of your model. Guessing at its structure...
var q = from ui in Context.UserImages
where ui.Invitations.Any(i => i.InviteeEmails.Any(e => e.InviteGuid = inviteGuid))
select ui.PageStyleCss;

from ui in db.UserImages
where (from inv in db.Invitations
join invEmails from InviteeEmails
on inv.InviteId equals invEmails.InviteId
where invEmails.InviteGUID == inviteGUID
select inv.UserImageId).Contains(ui.UserImageId)
select ui.PageStyleCss
(not sure if it compiles or not)
I have to assume there's a better way...this is pretty much a direct translation.

LINQ to SQL: Complicated query with aggregate data for a report from multiple tables for an ordering system

I want to convert the following query into LINQ syntax. I am having a great deal of trouble managing to get it to work. I actually tried starting from LINQ, but found that I might have better luck if I wrote it the other way around.
SELECT
pmt.guid,
pmt.sku,
pmt.name,
opt.color,
opt.size,
SUM(opt.qty) AS qtySold,
SUM(opt.qty * opt.itemprice) AS totalSales,
COUNT(omt.guid) AS betweenOrders
FROM
products_mainTable pmt
LEFT OUTER JOIN
orders_productsTable opt ON opt.products_mainTableGUID = pmt.guid
LEFT OUTER JOIN orders_mainTable omt ON omt.guid = opt.orders_mainTableGUID AND
(omt.flags & 1) = 1
GROUP BY
pmt.sku, opt.color, opt.size, pmt.guid, pmt.name
ORDER BY
pmt.sku
The end result is a table that shows me information about a product as you can see above.
How do I write this query, in LINQ form, using comprehension syntax ?
Additionally, I may want to add additional filters (to the orders_mainTable, for instance).
Here is one example that I tried to make work, and was fairly close but am not sure if it's the "correct" way, and was not able to group it by size and color from the orders_productsTable.
from pmt in products_mainTable
let Purchases =
from opt in pmt.orders_productsTable
where ((opt.orders_mainTable.flags & 1) == 1)
where ((opt.orders_mainTable.date_completedon > Convert.ToDateTime("01/01/2009 00:00:00")))
select opt
orderby pmt.sku
select new {
pmt.guid,
pmt.sku,
pmt.name,
pmt.price,
AvgPerOrder = Purchases.Average(p => p.qty).GetValueOrDefault(0),
QtySold = Purchases.Sum(p => p.qty).GetValueOrDefault(),
SoldFor = Purchases.Sum(p => p.itemprice * p.qty).GetValueOrDefault()
}
*Edit:
To be a little more explicit so you can understand what I am trying to do, here is some more explanation.
Products are stored in products_mainTable
Orders are stored in orders_mainTable
Products That Have Been Ordered are stored in orders_productsTable
I want to create several reports based on products, orders, etc. drilling into the data and finding meaningful bits to display to the end user.
In this instance, I am trying to show which products have been purchased over a period of time, and are the most popular. How many sold, for what price, and what is the breakout per order. Maybe not the best order, but I'm just experimenting and picked this one.
All of the tables have relationships to other tables. So from the product table, I can get to what orders ordered that product, etc.
The largest problem I am having, is understanding how LINQ works, especially with grouping, aggregate data, extensions, subqueries, etc. It's been fun, but it's starting to get frustrating because I am having difficulty finding detailed explanations on how to do this.

I'm also a beginner in LINQ. I don't know if this is the right way of grouping by several fields but I think you have to transform these grouping fields into a representing key. So, assuming that all your grouping fields are strings or ints you can make a key as follows:
var qry = from pmt in products_mainTable
join opt in orders_productsTable on pmt.guid equals opt.products_mainTableGUID
join omt in orders_mainTable on opt.orders_mainTableGUID equals omt.guid
where (opt.orders_mainTable.flags & 1) == 1
group omt by pmt.sku + opt.price + opt.size + pmt.guid + pmt.name into g
orderby g.sku
select new
{
g.FirstOrDefault().guid,
g.FirstOrDefault().sku,
g.FirstOrDefault().name,
g.FirstOrDefault().color,
g.FirstOrDefault().price,
AvgPerOrder = g.Average(p => p.qty).GetValueOrDefault(0),
QtySold = g.Sum(p => p.qty).GetValueOrDefault(),
SoldFor = g.Sum(p => p.itemprice * p.qty).GetValueOrDefault()
};
I didn't test this so please see if this helps you in any way.

Bruno, thank you so much for your assistance! The FirstOrDefault() was probably the largest help. Following some of what you did, and another resource I came up with the following that seems to work beautifully! This LINQ query below gave me nearly an exact replication of the SQL I posted above.
Here's the other resource I found on doing a LEFT OUTER JOIN in LINQ: Blog Post
Final Answer:
from pmt in products_mainTable
join opt in orders_productsTable on pmt.guid equals opt.products_mainTableGUID into tempProducts
from orderedProducts in tempProducts.DefaultIfEmpty()
join omt in orders_mainTable on orderedProducts.orders_mainTableGUID equals omt.guid into tempOrders
from ordersMain in tempOrders.DefaultIfEmpty()
group pmt by new { pmt.sku, orderedProducts.color, orderedProducts.size } into g
orderby g.FirstOrDefault().sku
select new {
g.FirstOrDefault().guid,
g.Key.sku,
g.Key.size,
QTY = g.FirstOrDefault().orders_productsTable.Sum(c => c.qty),
SUM = g.FirstOrDefault().orders_productsTable.Sum(c => c.itemprice * c.qty),
AVG = g.FirstOrDefault().orders_productsTable.Average(c => c.itemprice * c.qty),
Some = g.FirstOrDefault().orders_productsTable.Average(p => p.qty).GetValueOrDefault(0),
}

This was very helpful to me thanks. I had a similar issue I was trying to sort through only my case was much simpler as I didn't have any joins in it. I was simply trying to group one field, get the min of another, and the count. (min and count in the same query)
Here is the SQL I wanted to recreate in Linq syntax:
select t.Field1, min(t.Field2), COUNT(*)
from SomeTable t
group by t.Field1
order by t.Field1
Thanks to your post I eventually managed to come up with this:
from t in SomeTable
group t by new { t.Field1 } into g
orderby g.Key.Field1
select new
{
g.Key.Field1,
code = g.Min(c => c.Field2),
qty = g.Count()
}
Which creates the following SQL behind the scenes:
SELECT [t1].[Field1], [t1].[value] AS [code], [t1].[value2] AS [qty]
FROM (
SELECT MIN([t0].[Field2]) AS [value], COUNT(*) AS [value2], [t0].[Field1]
FROM [SomeTable] AS [t0]
GROUP BY [t0].[Field1]
) AS [t1]
ORDER BY [t1].[Field1]
Perfect, exactly what I was looking to do. The key for me was that you showed it possible to do this inside the new {} which is something I had never considered trying. This is huge, I now feel like I have a significantly better understanding going forward.