VB.NET 循环计数每本书的作者

Question

我想计算表格中每本书的作者数。 我得到的结果不正确。

表 Title_AuthorTest：

ISBN Au_ID2 书 1 - 作者 1， 书 2 - 作者 2， 书 2 - 作者 3，

结果 - 运行代码后：

2 本书有 1 个作者， 0 本书有 2 个作者， 0 本书有 3 个作者， ...

结果应该是：

1 本书有 1 个作者， 1 本书有 2 个作者， 0 本书有 3 个作者， ...

Imports System.Data
Imports System.Data.OleDb
Public Class Form1
    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        Dim BooksConnection As OleDbConnection
        Dim ISBNCommand As OleDbCommand
        Dim ISBNAdapter As OleDbDataAdapter
        Dim ISBNTable As DataTable
        'connect to books database
        BooksConnection = New OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=BooksDB.mdb;")
        BooksConnection.Open()
        'establish Command object
        ISBNCommand = New OleDbCommand("Select * from Title_AuthorTest ORDER BY ISBN", BooksConnection)
        'establish data adapter / data table
        ISBNAdapter = New OleDbDataAdapter()
        ISBNAdapter.SelectCommand = ISBNCommand
        ISBNTable = New DataTable()
        ISBNAdapter.Fill(ISBNTable)
        'count authors
        Dim Author As Integer
        Dim AuthorCount(10) As Integer 'make array
        Dim LastISBN As String
        LastISBN = ""

        'allow up to 10 authors per title
        For Author = 1 To 10
            AuthorCount(Author) = 0 'set authorcount for all the authors to zero
        Next Author
        'set to the first author
        Author = 1

        'check each row for repeated ISBN
        Dim MyRow As DataRow
        'LastISBN = ISBNTable.Rows(0)("ISBN").ToString 'get first ISBN
        For Each MyRow In ISBNTable.Rows 'loop the table
            If MyRow.Item("ISBN").Equals(LastISBN) Then
                'is ISBN repeated then add Author
                Author += 1
            Else
                'no more authors for this ISBN
                AuthorCount(Author) += 1
                Author = 1
                LastISBN = MyRow.Item("ISBN").ToString 'zet LastISBN op ISBN die je net hebt geconsulteerd
            End If
        Next
        'display results number of books with x authors
        For Author = 1 To 10
            ListBox1.Items.Add(Str(AuthorCount(Author)) + " Books with" + Str(Author) + " Authors")
        Next
        'dispose
        BooksConnection.Close()
        BooksConnection.Dispose()
        ISBNCommand.Dispose()
        ISBNAdapter.Dispose()
        ISBNTable.Dispose()
    End Sub
End Class

Answer 1

我将开始更改 SQL 命令并使用 GROUP BY

Dim cmdText = "SELECT ISBN, COUNT(ISBN) as NrAuthors from Title_AuthorTest 
               GROUP BY ISBN
               ORDER BY COUNT(ISBN) DESC"

ISBNCommand = New OleDbCommand(cmdText, BooksConnection)

现在您将所有图书按其 ISBN 分组，每条记录都包含 ISBN 和该 ISBN 的作者数量。
此时，您的代码可以大大简化

For Each MyRow as DataRow In ISBNTable.Rows 
    Dim numAuthors As Integer = Convert.ToInt32(MyRow("NrAuthors"))
    AuthorCount(numAuthors) += 1
Next

当然，这将允许最多 10 位作者撰写一本书。如果有一本书由 10 位以上的作者撰写，我们需要在索引 AuthorCount 数组之前进行检查，或者使用不同的结构更改我们存储此信息的方式