Question

我有一个名为DTR_Table的表，它有8列，即： EmployeeID，Date，MorningTime-In，MorningTime-Out，AfternoonTime-In，AfternoonTime-Out，UnderTime，Time-Rendered。我想阅读列＆＃34; AfternoonTime-In＆＃34;。以下是我的代码。它读取我的＆＃34; AfternoonTime-In＆＃34;字段，但它继续显示＆＃34;有行＆＃34;即使该栏目中没有任何内容。

我该如何解决这个问题？

Connect = New SqlConnection(ConnectionString)
Connect.Open()

Dim Query1 As String = "Select  [AfternoonTime-Out] From Table_DTR Where Date = @Date and EmployeeID = @EmpID "
Dim cmd1 As SqlCommand = New SqlCommand(Query1, Connect)
cmd1.Parameters.AddWithValue("@Date", DTRform.datetoday.Text)
cmd1.Parameters.AddWithValue("@EmpID", DTRform.DTRempID.Text)

Using Reader As SqlDataReader = cmd1.ExecuteReader()
    If Reader.HasRows Then
        MsgBox("Has rows")
        Reader.Close()

    Else
        MsgBox("empty")
    End If
End Using`

Answer 1

返回DataReader后，如果要从查询中提取值，则需要从中读取数据。

10-09 21:46:51.793 4867-4867/benlab.app1ben E/AndroidRuntime: FATAL EXCEPTION: main
                                                              Process: benlab.app1ben, PID: 4867
                                                              java.lang.RuntimeException: Unable to start activity ComponentInfo{benlab.app1ben/benlab.app1ben.game_page}: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.Random.nextInt()' on a null object reference
                                                                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2464)
                                                                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2524)
                                                                  at android.app.ActivityThread.access$900(ActivityThread.java:154)
                                                                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1391)
                                                                  at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                  at android.os.Looper.loop(Looper.java:234)
                                                                  at android.app.ActivityThread.main(ActivityThread.java:5526)
                                                                  at java.lang.reflect.Method.invoke(Native Method)
                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int java.util.Random.nextInt()' on a null object reference
                                                                  at benlab.app1ben.game_page.newProblem(game_page.java:47)
                                                                  at benlab.app1ben.game_page.onCreate(game_page.java:33)
                                                                  at android.app.Activity.performCreate(Activity.java:6285)
                                                                  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
                                                                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2417)
                                                                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2524) 
                                                                  at android.app.ActivityThread.access$900(ActivityThread.java:154) 
                                                                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1391) 
                                                                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                  at android.os.Looper.loop(Looper.java:234) 
                                                                  at android.app.ActivityThread.main(ActivityThread.java:5526) 
                                                                  at java.lang.reflect.Method.invoke(Native Method) 
                                                                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

请注意，我已使用更精确的Add更改了AddWithValue，并指定了参数类型。否则，您的代码将掌握数据库引擎决定使用的任何转换规则，以将传递给AddWithValue的字符串转换为DateTime。此转换通常会生成无效值，尤其是日期

如何读取数据库中的特定列？

1 个答案: