[英]Connecting to MySQL using VB.NET
我正在尝试制作一个非常简单的程序,它使用 VB.NET 连接到 MySQL 数据库。 我的程序只有一种形式和一种标签。 我将 MySQL 数据库设置为有一个表和一个字段(一个 VARCHAR)
称为“Tab1”。 我使用 PHPMyAdmin 手动将值“CLOSED”插入到 Tab1 字段中。 我希望我的程序将该字段的值更改为 OPEN/CLOSED 并且我还希望表单上的标签文本
单击时也会更改。
到目前为止,我已经在我的项目中添加了 MySQL.data 引用并添加了:
Imports System
Imports System.Data
Imports MySql.Data.MySqlClient
致 General_Declarations
我还声明了一个包含 MySQL 连接的全局变量:
Public Class Form1
Dim conn As New MySqlConnection
这是我连接到 MySQL 的函数:
Private Function Connect(ByVal server As String, ByRef user As String, ByRef password As String, ByRef database As String)
' Connection string with MySQL Info
conn.ConnectionString = "server=" + server + ";" _
& "user id=" + user + ";" _
& "password=" + password + ";" _
& "database=" + database + ";"
Try
conn.Open()
Return True
Catch ex As MySqlException
Return MsgBox(ex.Message)
End Try
End Function
我让程序在 Form_Load 上连接到 MySQL,如下所示:
Connect("db4free.net", "boomun", "*******", "boomdb")
它连接良好,但这里是我遇到麻烦的地方......我希望当我单击 Label1 时该字段从 CLOSED 更改为 OPEN。 它确实如此,但它不会在第二次单击时变回。 不知何故,我需要一个工作代码来更新表中的 Tab1 字段。
这是我到目前为止所拥有的:
Private Sub Label1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label1.Click
Dim myCommand As New MySqlCommand
Dim myAdapter As New MySqlDataAdapter
Dim myData As MySqlDataReader
Dim SQL As String
SQL = "SELECT * FROM `boomtable` WHERE `Tab1` = 'CLOSED'"
myCommand.Connection = conn
myCommand.CommandText = SQL
myAdapter.SelectCommand = myCommand
Try
myData = myCommand.ExecuteReader()
myData.Read()
If myData.HasRows = 0 Then
' *** UPDATE boomtable SET Tab1 = CLOSED WHERE Tab1 = OPEN ***
Label1.Text = "CLOSED"
myData.Close()
Else
Label1.Text = "OPEN"
' *** UPDATE boomtable SET Tab1 = OPEN WHERE Tab1 = CLOSED ***
myData.Close()
End If
Catch ex As MySqlException
MsgBox(ex.Message)
End Try
End Sub
那条 UPDATE 行对我不起作用......任何人都可以提供一个有效的代码吗? 谢谢我真的很感激!!
这是项目的全部代码:
Imports System
Imports System.Data
Imports MySql.Data.MySqlClient
Public Class Form1
Dim conn As New MySqlConnection
Private Function Connect(ByVal server As String, ByRef user As String, ByRef password As String, ByRef database As String)
' Connection string with MySQL Info
conn.ConnectionString = "server=" + server + ";" _
& "user id=" + user + ";" _
& "password=" + password + ";" _
& "database=" + database + ";"
Try
conn.Open()
Return True
Catch ex As MySqlException
Return MsgBox(ex.Message)
End Try
End Function
Private Sub Label1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label1.Click
Dim myCommand As New MySqlCommand
Dim myAdapter As New MySqlDataAdapter
Dim myData As MySqlDataReader
Dim SQL As String
SQL = "SELECT * FROM `boomtable` WHERE `Tab1` = 'CLOSED'"
myCommand.Connection = conn
myCommand.CommandText = SQL
myAdapter.SelectCommand = myCommand
Try
myData = myCommand.ExecuteReader()
myData.Read()
If myData.HasRows = 0 Then
' *** UPDATE boomtable SET Tab1 = CLOSED WHERE Tab1 = OPEN ***
Label1.Text = "CLOSED"
myData.Close()
Else
Label1.Text = "OPEN"
' *** UPDATE boomtable SET Tab1 = OPEN WHERE Tab1 = CLOSED ***
myData.Close()
End If
Catch ex As MySqlException
MsgBox(ex.Message)
End Try
End Sub
Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
Connect("db4free.net", "boomun", "boom123", "boomdb")
End Sub
End Class
请改为尝试以下语句:
UPDATE boomtable SET Tab1 = 'OPEN' WHERE Tab1 = 'CLOSED'
当然使用 ExecuteNonQuery 方法来执行语句。 如果您的意思是 OPEN 和 CLOSE 是变量,请改用参数(这样更安全)。
像这样...
Else
Label1.Text = "OPEN"
SQL = "UPDATE boomtable SET Tab1 = 'OPEN' WHERE Tab1 = 'CLOSED'"
myCommand.CommandText = SQL
myCommand.ExecuteNonQuery()
myData.Close()
End If
???
这里的一个错误是使用全局连接对象。 .Net 数据库连接使用一种称为连接池的功能,这意味着在大多数情况下,最好为每次调用数据库创建一个新的连接对象。 您可以做的是使用该全局来保留您的连接字符串。
Private Sub Label1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label1.Click
Dim SQL As String = "SELECT * FROM `boomtable` WHERE `Tab1` = 'CLOSED'"
Using rdrCn As New MySqlConnection(connectionString), _
rdrCmd As New MySqlCommand(SQL, rdrCn)
rdrCn.Open()
Using myData As MySqlDataReader = rdrCmd.ExecuteReader()
SQL = "UPDATE boomtable SET Tab1= @NewState WHERE Tab1 = @OldState"
Using updCn As New MySqlConnection(connectionString), _
updCmd As New MySqlCommand(SQL, updcn)
updCmd.Parameters.Add("@NewState", MySqlDbType.VarChar, 6)
updCmd.Parameters.Add("@OldState", MySqlDbType.VarChar, 6)
If myData.Read()
Label1.Text = "CLOSED"
updCmd.Parameters(0).Value = "CLOSED"
updCmd.Parameters(1).Value = "OPEN"
Else
Label1.Text = "OPEN"
Label1.Text = "CLOSED"
updCmd.Parameters(0).Value = "OPEN"
updCmd.Parameters(1).Value = "CLOSED"
End If
updCn.Open()
upCmd.ExecuteNonQuery()
End Using
End Using
End Using
End Sub
但这里更大的问题是,为什么要两次访问数据库?
Private Sub Label1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Label1.Click
Dim SQL As String = "UPDATE boomtable SET Tab1 = CASE WHEN Tab1='CLOSED' THEN 'OPEN' ELSE 'CLOSED' END;SELECT Tab1 FROM boomtable LIMIT 1;"
Using cn As New MySqlConnection(connectionString), _
cmd As New MySqlCommand(SQL)
cn.Open()
Label1.Text = CStr(cmd.ExecuteScalar())
End Using
End Sub
现在不是更整洁了吗?
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