Question

我有这段代码：

Function Get_Control_Station_Address(counter As Integer)
    Check_DB_Con() 'Check if the connection is okay
    SQL_Query = "SET @row_number = 0; " _
                & "SELECT hardware_add " _
                & "FROM (" _
                    & "SELECT " _
                    & "@row_number:=@row_number + 1 AS num, " _
                    & "hardware_add AS hardware_add " _
                    & "FROM teller_info" _
                & ") AS sub_query " _
                & "WHERE num = " & counter & ";"

    Dim MySQL_CMD As New MySqlCommand(SQL_Query, MysqlConn)

    Try
        MySQL_CMD.Connection.Open()
        MySQL_Reader = MySQL_CMD.ExecuteReader()

        While MySQL_Reader.Read
            MySQL_Result = MySQL_Reader("hardware_add")
        End While

        Return MySQL_Result
        MySQL_Reader.Close()
    Catch myerror As MySqlException
        Console.WriteLine("Failed to run query: " & myerror.Message)
        Return Nothing
    Finally
        MysqlConn.Close()
        MysqlConn.Dispose()
    End Try
End Function

我收到此错误：

A first chance exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll
A first chance exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll
A first chance exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll
A first chance exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll
A first chance exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll
A first chance exception of type 'MySql.Data.MySqlClient.MySqlException' occurred in MySql.Data.dll
Failed to run query: Fatal error encountered during command execution.

我确信这里有 no basic errors ，例如database connection error，作为证明我有这个功能，我100％肯定能够工作，它几乎与以前的功能不起作用，唯一的区别是查询。

Function Get_Control_Station_Total()
    Check_DB_Con() 'Check if the connection is okay
    SQL_Query = "SELECT COUNT(*) AS total_count FROM teller_info"
    Dim MySQL_CMD As New MySqlCommand(SQL_Query, MysqlConn)

    Try
        MySQL_CMD.Connection.Open()
        MySQL_Reader = MySQL_CMD.ExecuteReader()

        While MySQL_Reader.Read
            MySQL_Result = MySQL_Reader("total_count")
        End While

        Return MySQL_Result
        MySQL_Reader.Close()
    Catch myerror As MySqlException

        Return ("Failed to run query: " & myerror.Message)
    Finally
        MysqlConn.Close()
        MysqlConn.Dispose()
    End Try
End Function

所以仔细观察这个问题似乎是这个代码是错误的根源。

SQL_Query = "SET @row_number = '0'; " _
                & "SELECT hardware_add " _
                & "FROM (" _
                    & "SELECT " _
                    & "@row_number:=@row_number + 1 AS num, " _
                    & "hardware_add AS hardware_add " _
                    & "FROM teller_info" _
                & ") AS sub_query " _
                & "WHERE num = '" & counter & "';"

实际上我是使用heidiSQL直接重新创建该查询并且它工作得很好，所以我有点卡在这里，也许我使用 SET @row_number = '0'; 错了？

摘要：

我确定它不是MySQL连接问题
我确信MySQL查询正在运行（通过直接在HeidiSQL上测试）
获取数据的过程似乎正在起作用，因为我只是复制它从工作函数中获取数据的方式。

修改：通过关注@ Ken_White评论console.writeline的评论，我能够清楚地看到错误

这是错误

    MySql.Data.MySqlClient.MySqlException (0x80004005): Fatal error encountered during command execution. ---> MySql.Data.MySqlClient.MySqlException (0x80004005): Parameter '@row_number' must be defined.
   at MySql.Data.MySqlClient.Statement.SerializeParameter(MySqlParameterCollection parameters, MySqlPacket packet, String parmName, Int32 parameterIndex)
   at MySql.Data.MySqlClient.Statement.InternalBindParameters(String sql, MySqlParameterCollection parameters, MySqlPacket packet)
   at MySql.Data.MySqlClient.Statement.BindParameters()
   at MySql.Data.MySqlClient.PreparableStatement.Execute()
   at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior)
   at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader(CommandBehavior behavior)
   at MySql.Data.MySqlClient.MySqlCommand.ExecuteReader()
   at TCP_Client.Module_Database.Get_Control_Station_Address(Int32 counter) in C:\Users\xxxxx\xxx.vb:line 198

Answer 1

似乎添加 Allow User Variables = True到我的数据库连接解决了我的问题。

所以而不是

Public Sub Check_DB_Con()
    'Declare database credentials so we can refresh it in recheck timer
    server = My.Settings.DB_Server
    username = My.Settings.DB_Username
    password = My.Settings.DB_Password
    database = My.Settings.DB_Database

    'Connection String
    MysqlConn.ConnectionString = "server=" & server & ";" _
    & "user id=" & username & ";" _
    & "password=" & password & ";" _
    & "database=" & database 
End Sub

我添加了Allow User Variables=True，因此它将成为

Public Sub Check_DB_Con()
    'Declare database credentials so we can refresh it in recheck timer
    server = My.Settings.DB_Server
    username = My.Settings.DB_Username
    password = My.Settings.DB_Password
    database = My.Settings.DB_Database

    'Connection String
    MysqlConn.ConnectionString = "server=" & server & ";" _
    & "user id=" & username & ";" _
    & "password=" & password & ";" _
    & "database=" & database & ";Allow User Variables=True"
End Sub

Answer 2

不是在2步SQL中设置rn的值，而是使用交叉连接。我不认为该调用正确处理这两个语句。

 SQL_Query = "SELECT hardware_add " _
            & "FROM (" _
                & "SELECT " _
                & "@row_number:=@row_number + 1 AS num, " _
                & "hardware_add AS hardware_add " _
                & "FROM teller_info" _
                & "CROSS JOIN (SELECT @row_number:=0) AS t " _
            & ") AS sub_query " _
            & "WHERE num = '" & counter & "';"

堆栈示例：ROW_NUMBER() in MySQL

Answer 3

在连接字符串中添加 ConnectionTimeout 和默认命令超时为我解决了此问题。

例如

ConnectionTimeout=300000; default command timeout=300000;

“命令执行期间遇到致命错误”我正在使用SET和SELECT xxx FROM（SELECT xxx FROM xxx）

3 个答案: