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Implement an SQL query in LINQ

I'm trying implement the follow query in LINQ, but I don't find solution:
SQL:
SELECT COUNT(*) AS AmountMonths
FROM (SELECT SUBSTRING(CONVERT(NVARCHAR(12), pay_date, 112), 1, 6) AS Month
FROM #tmp
GROUP BY SUBSTRING(CONVERT(NVARCHAR(12), pay_date, 112), 1, 6)) AS AmountMonths
What I need is get the amounts of months in which the clients made payments, with the condition that there may be months in which no payments have been made.
In C# I tried the following:
int amountMonths = payDetail.GroupBy(x => Convert.ToDateTime(x.PayDate)).Count();
and
int amountMonths = payDetail.GroupBy(x => Convert.ToDateTime(x.PayDate).Month).Count();
But I am not getting the expected result.

(Assuming you're using EF Core)
You're almost there. You could do:
var amountMonths = context.AmountMonths.GroupBy(c => new { c.PayDate.Year, c.PayDate.Month }).Count();
This will translate to something like:
SELECT COUNT(*)
FROM (
SELECT DATEPART(year, [a].[PayDate]) AS [a]
FROM [AmountMonths] AS [a]
GROUP BY DATEPART(year, [a].[PayDate]), DATEPART(month, [a].[Pay_Date])
) AS [t]
which I'd find preferable over creating a string and chopping it up. EOMONTH isn't a standard mapped function, alas, otherwise it can be used to convert a date to month level granularity

Converting a SQL to Linq Query

Currently I have SQL query like
select tt.userId, count(tt.userId) from (SELECT userId,COUNT(userId) as cou
FROM [dbo].[users]
where createdTime> DATEADD(wk,-1,GETDATE())
group by userId,DATEPART(minute,createdTime)/5) tt group by tt.userId
Now I have the Data in the Data Table, I need to convert the above the query to LINQ and execute against the data table. I am unable to do so , can anybody help me out.
This is what query does, It groups the users into 5 minutes time slots and then counts the number of timeslots per user.
Note : I am not able to use Linqer to create the Linq queries because this table does not exist in the database, it's a virtual one created dynamically.

Bit complex query, giving my best to make it work.
var result = table.AsEnumerable().Where(u=> u.Field<DateTime>("createdTime") > DateTime.Now.AddDays(-7)) //subtract a week
.GroupBy(g=> new { userid = g.Field<string>("userId") , span = g.Field<DateTime>("createdTime").Minute })
.Select(g=> new { userid = g.Key.userid, count = g.Count()})
.GroupBy(g=> g.userid ).Select(s=> new {userid = s.Key, count = s.Count()});
Working Demo

This SQL can be rewritten like this
SELECT
COUNT(U.UserId),
U.[createdTime]
FROM USERS U WHERE createdTime> DATEADD(wk,-1,GETDATE())
GROUP BY U.UserId,
DATEPART(MONTH, U.[createdTime]),
DATEPART(DAY, U.[createdTime]),
DATEPART(HOUR, U.[createdTime]),
(DATEPART(MINUTE, U.[createdTime]) / 5)
And its corresponding Linq for DataTable would be
var users = myDataTable.AsEnumerable()
.Select(r=> new {
UserId = r.Field<int>("UserId"),
CreatedTime = r.Field<DateTime>("createdTime")
}).ToList();
var groupedUsersResult = from user in users where user.CreatedTime > user.CreatedTime.AddDays(-7) group user by
new {user.CreatedTime.Year,user.CreatedTime.Month,user.CreatedTime.Day,Minute=(user.CreatedTime.Minute/5),user.UserId}
into groupedUsers select groupedUsers;
Fiddle is here

I will suggest to use LINQPad4. It would be easy to do that and that will help you a lot in writing LINQ queries.
https://www.linqpad.net/

Proper use of DbFunctions.CreateDateTime()

I've got a database tabel 'DateExcluded' containing 3 int columns : Year, Month and Day, the latter being the daynumber of the month. I want their combination evaluated in an entity query, to retrieve all rows before one year from current date like so :
var l =
(from p in c.DateExcluded
where
DbFunctions.CreateDateTime(p.Year, p.Month, p.Day, null, null, null)
<= DateTime.Now.AddYears(1)
select p).ToList();
This query always returns 0 columns which it shouldn't. Wrong use of DbFunctions.CreateDateTime?

I've got a database tabel 'DateExcluded' containing 3 int columns : Year, Month and Day.
Don't ever create a column for each year, month and day
You're creating a non-sargable query, also know as the worst performing query you can create.
The correct way is to actually use a DateTime field. Then your query is just correct without any incorrect math possible.
var l =
(from p in c.DateExcluded
where
c.DateExcluded < DateTime.Now.AddYears(1).AddDay(1)
select p)
.ToList();

If you still want to use DbFunctions.CreateDateTime, avoid null values as it won't work correctly (see why in comment by Ole EH Dufour).
You should pass 0 instead of null, like this:
DbFunctions.CreateDateTime(p.Year, p.Month, p.Day, 0, 0, 0)

Entity Framework: Efficiently grouping by month

I've done a bit of research on this, and the best I've found so far is to use an Asenumerable on the whole dataset, so that the filtering occurs in linq to objects rather than on the DB. I'm using the latest EF.
My working (but very slow) code is:
var trendData =
from d in ExpenseItemsViewableDirect.AsEnumerable()
group d by new {Period = d.Er_Approved_Date.Year.ToString() + "-" + d.Er_Approved_Date.Month.ToString("00") } into g
select new
{
Period = g.Key.Period,
Total = g.Sum(x => x.Item_Amount),
AveragePerTrans = Math.Round(g.Average(x => x.Item_Amount),2)
};
This gives me months in format YYYY-MM, along with the total amount and average amount. However it takes several minutes every time.
My other workaround is to do an update query in SQL so I have a YYYYMM field to group natively by. Changing the DB isn't an easy fix however so any suggestions would be appreciated.
The thread I found the above code idea (http://stackoverflow.com/questions/1059737/group-by-weeks-in-linq-to-entities) mentions 'waiting until .NET 4.0'. Is there anything recently introduced that helps in this situation?

The reason for poor performance is that the whole table is fetched into memory (AsEnumerable()). You can group then by Year and Month like this
var trendData =
(from d in ExpenseItemsViewableDirect
group d by new {
Year = d.Er_Approved_Date.Year,
Month = d.Er_Approved_Date.Month
} into g
select new
{
Year = g.Key.Year,
Month = g.Key.Month,
Total = g.Sum(x => x.Item_Amount),
AveragePerTrans = Math.Round(g.Average(x => x.Item_Amount),2)
}
).AsEnumerable()
.Select(g=>new {
Period = g.Year + "-" + g.Month,
Total = g.Total,
AveragePerTrans = g.AveragePerTrans
});
edit
The original query, from my response, was trying to do a concatenation between an int and a string, which is not translatable by EF into SQL statements. I could use SqlFunctions class, but the query it gets kind ugly. So I added AsEnumerable() after the grouping is made, which means that EF will execute the group query on server, will get the year, month, etc, but the custom projection is made over objects (what follows after AsEnumerable()).

When it comes to group by month i prefer to do this task in this way:
var sqlMinDate = (DateTime) SqlDateTime.MinValue;
var trendData = ExpenseItemsViewableDirect
.GroupBy(x => SqlFunctions.DateAdd("month", SqlFunctions.DateDiff("month", sqlMinDate, x.Er_Approved_Date), sqlMinDate))
.Select(x => new
{
Period = g.Key // DateTime type
})
As it keeps datetime type in the grouping result.

Similarly to what cryss wrote, I am doing the following for EF. Note we have to use EntityFunctions to be able to call all DB providers supported by EF. SqlFunctions only works for SQLServer.
var sqlMinDate = (DateTime) SqlDateTime.MinValue;
(from x in ExpenseItemsViewableDirect
let month = EntityFunctions.AddMonths(sqlMinDate, EntityFunctions.DiffMonths(sqlMinDate, x.Er_Approved_Date))
group d by month
into g
select new
{
Period = g.Key,
Total = g.Sum(x => x.Item_Amount),
AveragePerTrans = Math.Round(g.Average(x => x.Item_Amount),2)
}).Dump();
A taste of generated SQL (from a similar schema):
-- Region Parameters
DECLARE #p__linq__0 DateTime2 = '1753-01-01 00:00:00.0000000'
DECLARE #p__linq__1 DateTime2 = '1753-01-01 00:00:00.0000000'
-- EndRegion
SELECT
1 AS [C1],
[GroupBy1].[K1] AS [C2],
[GroupBy1].[A1] AS [C3]
FROM ( SELECT
[Project1].[C1] AS [K1],
FROM ( SELECT
DATEADD (month, DATEDIFF (month, #p__linq__1, [Extent1].[CreationDate]), #p__linq__0) AS [C1]
FROM [YourTable] AS [Extent1]
) AS [Project1]
GROUP BY [Project1].[C1]
) AS [GroupBy1]

SQL; Plus + or Minus – Instead of Dates (ASP.NET-C#)

I am Working On ASP.NET_C# Application
I am pulling out some Dates form SQL on a Grid View.
But instead of those Dates, I like to see how mush “Plus +” or “minus –“ It is form Today’s Date.
For Example
Today’s Date is = 11/02/2012
If the Date from SQL is 07/02/2012, I like my Grid to show -4
Or
If the Date from SQL is 17/02/2012, I like my Grid to show +6
Please Help me, How do I go about doing that….
This is My SQL Query; That I am working With
SELECT MAX(AmountPay.DateUpto) AS [ PaidUpTo], TimeTable.Name, TimeTable.Ref, TimeTable.Time11to12 FROM AmountPay FULL OUTER JOIN TimeTable ON AmountPay.Ref = TimeTable.Ref WHERE (TimeTable.Time11to12 = #Time11to12) GROUP BY TimeTable.Name, TimeTable.Ref, TimeTable.Time11to12
AmountPay.DateUpto is the Date I Like see as + or -
I have Been Suggested Datediff Function; and I have Used datediff function for working out things like Age from Date of Birth but I can't work out; how I can use this in the Case
Thanks in Advance

You can use Timespan
DateTime oldDate = DateTime.Now.AddDays(-4);
DateTime today = DateTime.Now;
TimeSpan span = oldDate.Subtract(today);
string diff = span.Days.ToString();

Since you are using MySQL, your select statement should be like:
SELECT DATEDIFF(CURDATE(), MAX(AmountPay.DateUpto)) AS [ PaidUpTo],
TimeTable.Name,
......
DATEDIFF will give you values like -1, 1, -5 ...and your DataGridView should display those values just fine.

SELECT DATEDIFF(day,GetDate(), MAX(AmountPay.DateUpto)) AS [ PaidUpTo], TimeTable.Name, TimeTable.Ref, TimeTable.Time11to12 FROM AmountPay FULL OUTER JOIN TimeTable ON AmountPay.Ref = TimeTable.Ref WHERE (TimeTable.Time11to12 = #Time11to12) GROUP BY TimeTable.Name, TimeTable.Ref, TimeTable.Time11to12