关于ado.net

本文介绍了关于ado的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！ 问题描述

如何检查用户是否存在于数据库中

how to check whether the user exists in the database

推荐答案

为选择用户编写查询，如果返回空则数据库中没有用户 write query for select user, if it return empty there is no user else have user in database

公共表1_string作为字符串= String.Empty 将公共Table1_command作为OleDb.OleDbCommand 将公共Table1_dataadapter作为OleDb.OleDbDataAdapter 将公共Table1_commandbuilder作为OleDb.OleDbCommandBuilder 公用Table1_table作为新数据表 将公共Table1_bindingsource作为新的BindingSource 以OleDb.OleDbDataReader的形式公开Table1_data_reader 将公共db18_connection作为新的OleDb.OleDbConnection("Provider = Microsoft.Jet.OLEDB.4.0; Data Source = C:\ 1 \ db18.mdb; Persist Security Info = True") 私有子Button1_Click(ByVal发送者为System.Object，ByVal e为System.EventArgs)处理Button1.Click 昏暗的UserName_variable为String = String.Empty Table1_string ="SELECT * FROM Table1 WHERE UserName = @UserName" Table1_command =新的OleDb.OleDbCommand(Table1_string，db18_connection) 使用Table1_command Table1_command.Parameters.Add(New OleDb.OleDbParameter("@ UserName"，OleDb.OleDbType.Char，50)).Value = Me.UserNameTextBox.Text 结尾为 如果Table1_command.Connection.State = ConnectionState.Closed然后 Table1_command.Connection.Open() 如果结束 Table1_data_reader = Table1_command.ExecuteReader() 如果是Table1_data_reader.Read然后 如果不是TypeOf Table1_data_reader("UserName")是DBNull，则 UserName_variable = CStr(Table1_data_reader("UserName")) 如果结束 Form3.Show() 其他 MsgBox(密码错误") 如果结束 Table1_data_reader.Close() Table1_command.Connection.Close() 结束Sub< big></big> Public Table1_string As String = String.Empty Public Table1_command As OleDb.OleDbCommand Public Table1_dataadapter As OleDb.OleDbDataAdapter Public Table1_commandbuilder As OleDb.OleDbCommandBuilder Public Table1_table As New DataTable Public Table1_bindingsource As New BindingSource Public Table1_data_reader As OleDb.OleDbDataReader Public db18_connection As New OleDb.OleDbConnection("Provider=Microsoft.Jet.OLEDB.4.0;Data Source=C:\1\db18.mdb;Persist Security Info=True") Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click Dim UserName_variable As String = String.Empty Table1_string = "SELECT * FROM Table1 WHERE UserName = @UserName" Table1_command = New OleDb.OleDbCommand(Table1_string, db18_connection) With Table1_command Table1_command.Parameters.Add(New OleDb.OleDbParameter("@UserName", OleDb.OleDbType.Char, 50)).Value = Me.UserNameTextBox.Text End With If Table1_command.Connection.State = ConnectionState.Closed Then Table1_command.Connection.Open() End If Table1_data_reader = Table1_command.ExecuteReader() If Table1_data_reader.Read Then If Not TypeOf Table1_data_reader("UserName") Is DBNull Then UserName_variable = CStr(Table1_data_reader("UserName")) End If Form3.Show() Else MsgBox("Incorrect Password") End If Table1_data_reader.Close() Table1_command.Connection.Close() End Sub<big></big>