(SQL, VB.NET) How Do I select multiple values from one row and assign them to variables? - sql

Question

I am writing a SELECT query to use in my project. So far, I have
Dim conn As New OleDbConnection
Dim StudentID, GradeID, SubjectID As Integer
Dim YourGrade(4), YourSubject(4) As String
conn.ConnectionString = "Provider=Microsoft.ACE.OLEDB.12.0;Data Source =H:\Year 13 Computer Science\Project\Usernames and Passwords.accdb"
conn.Open()
Dim sql = "Select * From Grades where StudentID =" & CurrentID
Dim cmd As New OleDbCommand(sql, conn)
Dim dr As OleDbDataReader = cmd.ExecuteReader
While dr.Read
StudentID = dr("StudentID")
GradeID = dr("GradeID")
SubjectID = dr("SubjectID")
End While
My issue is that I need to be able to have a dynamic number of SubjectIDs and GradeIDs to be selected, in case a student is taking more or fewer subjects than the normal three.
My Query produces:
StudentID GradeID SubjectID
1 2 1
1 4 13
1 3 19
the CurrentID is "1" for the purposes of this.
Each GradeID and SubjectID corresponds to values in other tables which I can work on later.
I need to be able to have each of those three Grade IDs in a separate value, an array could be used but I don't know how to code it. I attempted it earlier as shown by the "YourGrade(4), YourSubject(4)".
I intend to use the data to fill out a Data Grid.

Answer 1

Create a domain object for "Student" and then load the records into a list of Student Objects.
I don't know VB.net, but the equivalent Domain object in C# would look like this:
public class Student
{
public int StudentId { get; set; }
public int GradeId { get; set; }
public int SubjectId { get; set; }
}
And then the code to loop through the dataReader and populate the list:
List<Student> results = new List<Student>();
while (dr.Read())
{
results.Add(new Student()
{
StudentId = Convert.ToInt32(dr["StudentID"]),
GradeId = Convert.ToInt32(dr["GradeId"]),
SubjectId = Convert.ToInt32(dr["SubjectId"])
});
}
-- Edit 2/2/2017 --
Turns out there are free converters on the web. These are the VB.net equivalents to the snippets above.
Class:
Public Class Student
Public Property StudentId() As Integer
Get
Return m_StudentId
End Get
Set
m_StudentId = Value
End Set
End Property
Private m_StudentId As Integer
Public Property GradeId() As Integer
Get
Return m_GradeId
End Get
Set
m_GradeId = Value
End Set
End Property
Private m_GradeId As Integer
Public Property SubjectId() As Integer
Get
Return m_SubjectId
End Get
Set
m_SubjectId = Value
End Set
End Property
Private m_SubjectId As Integer
End Class
Database Code:
Dim results As New List(Of Student)()
While dr.Read()
results.Add(New Student() With { _
Key .StudentId = Convert.ToInt32(dr("StudentID")), _
Key .GradeId = Convert.ToInt32(dr("GradeId")), _
Key .SubjectId = Convert.ToInt32(dr("SubjectId")) _
})
End While

Answer 2

I did some digging, found out that to fill out a datagrid in the way I was attempting to, the simplest way is to use an INNER JOIN SQL statement. In SQL the query is:
SELECT Students.FirstName, Students.LastName, Subjects.SubjectName, GradeVals.Grade
FROM GradeVals INNER JOIN (Students INNER JOIN (Subjects INNER JOIN Grades ON Subjects.SubjectID = Grades.SubjectID) ON Students.StudentID = Grades.StudentID) ON GradeVals.GradeID = Grades.GradeID;
That's using my Table names and such as.