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Cascade Dropdown and set Detail value properly

I have the following cascade relationships between Parent and Child categories and keep the ChildCategoryId in the Personel table. When creating a new Personel record there are 2 cascade Dropdownlist and child records are listed according to their parents. Only ChildCategoryId is saved to the Personel table e.g. when select Education and Sales, the ChildCategoryId is 3. However, when selecting a parent record that has no child i.e. Family or Culture, I have no idea what value should be saved to the Personel table.
Of course I can add two "N/A" records to the Child table for Family or Culture records and save their Ids to the Personel table, but I am not sure if it is a good idea. Because when I list the Child category name, "N/A" will be displayed on the details page.
Is there a best practices for this kind of scenarios?
Personel:
Id | Name | ChildCategoryId |
----------------------------------
1001 | John | 1 |
1002 | Mary | 2 |
1003 | Bill | 3 |
1004 | Jose | 4 |
1005 | Anna | 5 |
ChildCategory:
Id | Name | ParentCategoryId |
-----------------------------------
1 | IT | 101 |
2 | Finance | 102 |
3 | Sales | 102 |
4 | PR | 103 |
5 | HR | 103 |
ParentCategory:
Id | Name |
------------------
101 | Health |
102 | Education |
103 | Sport |
104 | Family |
105 | Culture |

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)));
}

MSSQL Query omitting specific row value?

I have the following query to retrieve the customer sales report based on customer ID. It works, but how can I omit the customer name being duplicated?
select Cu.CustomerName, Cu.City, pd.pname,
Cs.qty, Cs.totalAmount, Cs.payed, Cs.credit, Cs.CreditEndDate
from Customer Cu
inner join CreditSales Cs on Cu.ID=Cs.CustomerID
left join Product pd on pd.pid=Cs.pid
where Cu.ID=6
order by Cu.CustomerName
The result is:
+--------+-------+---------------+----+----------+----------+----------+------------+
| Halima | Jimma | Mouse | 1 | 345.00 | 345.00 | 0.00 | 2015-08-29 |
| Halima | Jimma | Mobile | 10 | 92000.00 | 40000.00 | 52000.00 | 2015-08-23 |
| Halima | Jimma | Iphone | 2 | 13800.00 | 6500.00 | 7300.00 | 2015-08-28 |
| Halima | Jimma | Tape Recorder | 10 | 5175.00 | 4000.00 | 1175.00 | 2015-10-30 |
+--------+-------+---------------+----+----------+----------+----------+------------+
But I need like this:
+--------+-------+---------------+----+----------+----------+----------+------------+
| Halima | Jimma | Mouse | 1 | 345.00 | 345.00 | 0.00 | 2015-08-29 |
| | Jimma | Mobile | 10 | 92000.00 | 40000.00 | 52000.00 | 2015-08-23 |
| | Jimma | Iphone | 2 | 13800.00 | 6500.00 | 7300.00 | 2015-08-28 |
| | Jimma | Tape Recorder | 10 | 5175.00 | 4000.00 | 1175.00 | 2015-10-30 |
+--------+-------+---------------+----+----------+----------+----------+------------+
That means the customer name should displayed only one instance.
Any help? Thanks in Advance.

Based on Milen Pavlov response, you can try this:
SELECT CASE WHEN ROW_NUMBER() over (partition by [CustomerName] order by [CustomerName]) = 1 THEN [CustomerName] ELSE '' END
FROM Customer
ORDER BY [CustomerName]

This could do the job (I need you have to add an ordering in the over() clause):
select case when a.rn > 1 then '' else a.CustomerName end as CustomerName, a.City, a.pname, a.qty, a.totalAmount, a.payed, a.credit, a.CreditEndDate
from
(
select Cu.CustomerName, Cu.City, pd.pname,
Cs.qty, Cs.totalAmount, Cs.payed, Cs.credit, Cs.CreditEndDate, ROW_NUMBER() over (partition by Cu.CustomerName order by Cs.CreditEndDate) as rn
from Customer Cu
inner join CreditSales Cs on Cu.ID=Cs.CustomerID
left join Product pd on pd.pid=Cs.pid
where Cu.ID=6
) a
order by a.CustomerName

don't think it's a job for SQL but if you insists you can create two tables the first one with all the rows and columns and than delete the first row, and the other table with the first row and just make a left join and make the join on all the fields.

Divide Crystal Report Vertically in Two Parts

I have to SP, from which i want to generate Report. I need output in two different Columns on one Detail Sections. I have tried Format with Multiple Columns, but it do not full fill my needs..
-- My Data
-- Student Record
+------+------------+
| Name | FatherName |
+------+------------+
| A | B |
| C | D |
| E | F |
+------+------------+
--Teacher Record
+------+------------+
|Name | FatherName |
+------+------------+
| W | X |
| Y | Z |
+------+------------+
My required out will look like below ...
Students Record Teachers Record
+------+------------+ +------+------------+
| Name | FatherName | | Name | FatherName |
+------+------------+ +------+------------+
| A | B | | W | X |
| C | D | | Y | Z |
| E | F | +------+------------+
+------+------------+
Is there any way to get this output in one Detail Section. Student and Teacher have different number of records, one have 10 record 2nd have 5 record..
Any Help...

You can't do this in one detail section in Crystal Reports.
However, you can develop a report for each of these tables separately, then add the two reports side-by-side as sub-reports into a new report - this should achieve your objective.

I think you can do it by right-clicking the details section then Insert->subreport, one for Teachers record and one for the Students record, then editing these subreports with their respective data source.

Gridview and Select query

I am working with gridview and tables, I am using c# and asp.net. Assuming I have 2 tables, lets just name it Profiles and Info.
In table profiles let's say I have fields with p_Id, FirstName, LastName and in Transaction I have I_Id, childsID, fathersID, mothersID.
This is what my tables looks like:
Profile
| p_Id | FirstName | LastName |
| 1 | Jack | Cole |
| 2 | Cynthia | Cole |
| 3 | Robert | Cole |
Info
| I_Id | childsID | fathersID | mothersID |
| 1 | 1 | 3 | 2 |
Now in my gridview I have to display FirstName, LastName, Father and Mother. But I need to display their names not their ID's.
For now my gridview looks like this:
First Name | Last Name | Father| Mother |
Jack | Cole | 3 | 2 |
what I want is like this:
First Name | Last Name | Father | Mother |
Jack | Cole | Robert Cole | Cynthia Cole |

try this
select p.firstName, p.LastName
, (select sp.FirstName+' '+sp.Lastname FROM profile sp WHERE sp.p_id=i.fathersid) as father
, (select sp.FirstName+' '+sp.Lastname FROM profile sp WHERE sp.p_id=i.mothersid) as mother
from info i
inner join profile p ON (p.p_id=i.childsID)