Develop Reference

LINQ: equivalent for string_agg from PostgreSql

I need to concatenate multiple columns values to single value (separated with coma for example). I can do it by string_agg(u.name, ',') as Users in PostgreSql. I tried do it in linq query syntax but I failed - all time I just get list with split values instead of one row with values concatenated in one field.
Let's assume that I have only three tables:
Doc User Doc_User
+----+--------------------+ +----+-----------+ +----+-----------+
| Id | Path | | Id | Name | | DocId | UserId |
+----+--------------------+ +----+-----------+ +----+-----------+
| 1 | "C:\\Files\\A.txt" | | 1 | "Adam" | | 1 | 1 |
| 2 | "C:\\Files\\B.txt" | | 2 | "Benny" | | 1 | 2 |
| 3 | "C:\\Files\\C.txt" | | 3 | "Charlie" | | 2 | 1 |
+----+--------------------+ +----+-----------+ | 2 | 2 |
| 2 | 3 |
+-------+--------+
At the start I was trying simple join:
var model = (from d in db.Docs
join du in db.DU on d.Id equals du.DocId
join u in db.Users on du.UserId equals u.Id
select new DataModel() { DocPath = d.Path, UserName = u.Name }).ToList();
As I suspected I got list with separated rows:
C:\Files\A.txt | Adam
C:\Files\A.txt | Benny
C:\Files\B.txt | Adam
C:\Files\B.txt | Benny
C:\Files\B.txt | Charlie
But what I want to obtain is:
C:\Files\A.txt | Adam, Benny
C:\Files\B.txt | Adam, Benny, Charlie
string.Join() is not recognized by EF and I can't use it in linq queries, Aggregate() not working too. How can I achieve this?
I've prepared example for play with it: .Net Fiddle.

The code below groups the common documents by path using LINQ and then joins the grouped document's UserNames, using string.Join. I also think you don't need ToList() in this line select new DataModel() { DocPath = d.Path, UserName = u.Name }).ToList(); if you are going to use this solution because we are going to use LINQ again.
var grouped = model.GroupBy(x => x.DocPath);
foreach (var iGrouped in grouped){
string.Join(",",iGrouped.Select(x => x.UserName));
Console.WriteLine(iGrouped.Key + " | " + string.Join(",",iGrouped.Select(x => x.UserName)));
}

MySQL query for search filter from multiple tables

I have multiple tables and i have a search ASP.NET page with a GridView for the results.
The GridView must contain Name, School, State, Country
I have multiple tables that only contains the data:
index_States
indexID| State
----------------
1 | state1
2 | state2
3 | state3
index_Contries
indexID| Country
----------------
1 | country1
2 | country2
3 | country3
index_Schools
indexID| School
----------------
1 | school1
2 | school2
3 | school3
Then i have the tables that contains the indexID as reference
General_Info
idKey | Name
--------------
1 | John
2 | McClane
3 | Jr.
Academic_XP
id | idSchool | idState | idCountry | idKey
--------------------------------------------
1 | 1 | 3 | 20 | 2
2 | 1 | 5 | 146 | 3
3 | 2 | 1 | 65 | 9
And THEN I have the table that contains UserType as only certain type of user will be searched
Users
id | UserType | idKey
-----------------------
1 | 1 | 1
2 | 3 | 2
3 | 3 | 3
4 | 1 | 4
I've already tried multiple queries but none seem to be working.
Last query that seem to be working was with INNER JOIN
SELECT Name, State
FROM General_Info A, Academic_XP B
INNER JOIN index_States D ON B.idState = D.indexID
GROUP BY A.id;
but it doesn't work as soon as I add a second INNER JOIN or a WHERE clause.
SELECT Name, State
FROM General_Info A, Academic_XP B, Users
INNER JOIN index_States D ON B.idState = D.indexID
INNER JOIN index_School E ON B.idSchool = E.indexID
GROUP BY A.id
WHERE Users.UserType = 3;
I don't know how can I do that.
So i guess the question is
How can I made a query that returns from all those tables something like this?
Name | State | School | Country
---------------------------------------
McClane | state3 | school1 | country20
Jr. | state1 | school5 | country146
Note that McClane and Jr. are both UserType 3.
I will appreciate any help.

You are producing a cartesian product between tables without joins. I think this is what you're looking for using additional JOINs:
SELECT DISTINCT
G.Name,
S.State,
C.Country,
SC.School
FROM Academic_XP A
JOIN Users U ON A.idKey = U.idKey
JOIN General_Info G ON A.id = G.idKey
JOIN Index_States S ON A.idState = S.indexID
JOIN Index_Contries C ON A.idCountry = C.indexID
JOIN Index_Schools SC ON A.idSchool = SC.indexID
WHERE U.UserType = 3
If some tables don't have matching keys, you'll need to use an OUTER JOIN.
A Visual Explanation of SQL Joins

Convert SQL containing IN statement to chainable LINQ

I am trying to convert an MSSQL statement into LINQ. The overall functionality is broken up into many LINQ statements which are combined together, so the result must return type IQueryable.
Included in this question is an example of the schema and data. I am trying to see if the most recent (by DateCreated) value of Number is equal to 400, if so return the Table1Id.
The following is the MSSQL statement.
select * from Table1 t1
where 400 in (
select top 1 t2.Number
from Table2 t2
where t2.Table1Id = t1.id
order by t2.DateCreated desc
)
The LINQ I have come up with so far is:
//initial query
var query1 = _table1Repository.Table
//chain our query
var query2 = from t1 in query1
where ((from t2 in _table2Repository.Table
where t2.Table1Id == t1.Id
orderby t2.DateCreated descending
select t2.Number)
.Take(1)).Contains(400)
select t1;
//execute the query (may be more queries between last query and this)
var queryResult = query2.ToList();
Though upon executing the following error is thrown:
Unable to create a constant value of type 'Project.Domain.Table2'. Only primitive types or enumeration types are supported in this context.
Table 1 data
| Id |
|----|
| 1 |
| 2 |
| 3 |
Table 2 data
| Id | DateCreated | Table1Id | Number |
|----|------------ |-----------|---------|
| 1 | 1/1/2014 | 1 | 100 |
| 2 | 2/1/2014 | 1 | 200 |
| 3 | 3/1/2014 | 1 | 300 |
| 4 | 1/1/2014 | 2 | 200 |
| 5 | 2/1/2014 | 2 | 300 |
| 6 | 3/1/2014 | 2 | 400 |
| 7 | 1/1/2014 | 3 | 400 |
| 8 | 2/1/2014 | 3 | 300 |
| 9 | 3/1/2014 | 3 | 200 |
Expected result
| Id |
|----|
| 2 |

Assuming you still want to use the In keyword, the Lambda version would look like:
var intList = new List<int>(1) { 400 };
var test = _table1Repository.Table
.Where(t1 => intList.Contains(_table2Repository.Table
.Where(t2 => t2.Table1Id == t1.id)
.OrderByDescending(t2 => t2.Number)
.First()));
The only Caveat is that if there are no values in Table2, First() with throw an exception.
An example of mix'd Linq and Lambda:
var query1 = _table1Repository.Table
.OrderBy(t1 => t1.id);
var query2 = (from t1 in query1
select new { Id = t1.Id, Number = number } );
var query3 = query2.ToList();

linq query sum products across multiple collections

I have two datasets that look like this:
+------------------------------------+
| Products |
+------------------------------------+
| Id | Name | Price |
+------------------------------------+
| 1 | apples | 1.00 |
| 2 | oranges | 2.00 |
| 3 | pomengrate | 3.00 |
+------------------------------------+
+-------------------------------+
| Sales |
+-------------------------------+
| CustId | ProductId | Quantity |
+-------------------------------+
| 1 | 1 | 5 |
| 1 | 2 | 4 |
| 1 | 3 | 2 |
| 2 | 1 | 8 |
| 2 | 3 | 7 |
+-------------------------------+
I want to get the amount that each customer is spending, essentially ending up with a result like this:
+----------------+
| CustId | Total |
+----------------+
| 1 | 19.00 |
| 2 | 29.00 |
+----------------+
I can do a sum across a single table but the Sum method in LINQ takes a lambda with only one argument: the reference to the table that the values being summed are in. These values are in different tables. How do I sum these together?

var totals =
from sale in Sales
group sale by sale.CustId into custSales
select new
{
CustId = custSales.Key,
Total = (
from custSale in custSales
select custSale.Product.Price *
custSale.Quantity)
.Sum()
};

Give this a shot. It gives the results you were looking for:
var results = sales.Join(products,
sale => sale.ProductID,
product => product.ID,
(sale, product) => new { CustID = sale.CustID, Total = sale.Quantity * product.Price })
.GroupBy(r => r.CustID)
.Select(g => new { CustID = g.Key, Total = g.Sum(gt => gt.Total) });

For completeness sake, here's the query syntax version (utilizing joins as opposed to a subselect):
var totals =
from sale in sales
join prod in product on sale.ProductId equals prod.Id
let saleProds = new { sale.CustId, Total = prod.Price * sale.Quantity }
group saleProds by saleProds.CustId into custSale
select new { Customer = custSale.Key, Total = custSale.Sum(tr => tr.Total) };
The key part is that you need to somehow transform the joined collection results (sale and prod) into a single entity that can then be grouped by.

How to filter out similar rows (equal on certain columns) based on other column data

How would I select all rows distinct on Forename and Surname and where there's a duplicate select the one with the higher SomeDate, then Id if still duplicates e.g.
For:
| Id | Forename | Surname | SomeDate |
----------------------------------------
| 1 | Bill | Power | 2011-01-01 |
| 2 | James | Joyce | 2011-02-01 |
| 3 | Peter | Lennon | 2011-03-01 |
| 4 | John | Sellers | 2011-04-01 |
| 5 | James | Joyce | 2011-05-01 |
| 6 | Peter | Lennon | 2011-03-01 |
Results in:
| Id | Forename | Surname | SomeDate |
----------------------------------------
| 1 | Bill | Power | 2011-01-01 |
| 4 | John | Sellers | 2011-04-01 |
| 5 | James | Joyce | 2011-05-01 |
| 6 | Peter | Lennon | 2011-03-01 |
How could I achieve this in
T-SQL
From a DataTable using C#

Assuming SQL Server 2005+, use:
SELECT x.id,
x.forename,
x.surname,
x.somedate
FROM (SELECT t.id,
t.forename,
t.surname,
t.somedate,
ROW_NUMBER() OVER (PARTITION BY t.forename, t.surname
ORDER BY t.somedate DESC, t.id DESC) AS rank
FROM TABLE t_ x
WHERE x.rank = 1
A risky approach would be:
SELECT MAX(t.id) AS id,
t.forename,
t.surname,
MAX(t.somedate) AS somedate
FROM TABLE t
GROUP BY t.forename, t.surname

I'd tend to use subselects for the non-grouped values.
SELECT Forename, Surname,
(SELECT TOP 1 Id FROM myTable mt WHERE mt.Forename = m.Forename AND mt.Surname = m.Surname
ORDER BY m.SomeDate DESC) AS Id
(SELECT TOP 1 SomeDate FROM myTable mt WHERE mt.Forename = m.Forename AND mt.Surname = m.Surname
ORDER BY m.SomeDate DESC) AS SomeDate
FROM myTable m
GROUP BY Forename, Surname
Or you can filter it in the WHERE clause:
SELECT Id, Forename, Surname, SomeDate
FROM myTable m
WHERE m.Id = (SELECT TOP 1 Id FROM myTable mt WHERE mt.Forename = m.Forename AND mt.Surname = m.Surname
ORDER BY m.SomeDate DESC)
I'm afraid neither will be terribly efficient, but index toning will ameliate that if needed.
For a datatable example, you'd do essentially the same thing.
var recs = from record in dataTable
where record.Id ==
(from rec in dataTable
where rec.Forename == record.Forename && rec.Surname == record.Surname
orderby rec.SomeDate descending
select rec.Id).First()
select record;