Visual Studio 2012中的SQL查询错误

Question

我正在努力编写可以在工作中使用的东西，并且正在自行学习。我遇到了一个我找不到解决方案的问题，虽然我很可能只是不熟悉正确的术语。

我已将Visual Studio项目附加到SQL数据库，并试图执行搜索，我可以在三个文本框中的任何一个中输入信息，并将相应地过滤返回的结果。这里是用户搜索查询：

SELECT  First_Name AS [First Name], Middle_Name AS [Middle Name], Last_Name AS [Last Name], Users_ID AS [Unique ID] 
FROM   Users 
WHERE  (First_Name LIKE '%' + @firstName + '%') AND (Last_Name LIKE '%' + @lastName + '%') AND (Users_ID LIKE '%' + @uniqueID + '%') 
ORDER BY [Last Name] 

这是我用来调用搜索的SUB。我唯一遇到的问题是用户搜索。

Private Sub searchButton_Click(sender As Object, e As EventArgs) Handles searchButton.Click 
    'Clear all variables when the search button is clicked. This allows Search to start with a clean' 
    'slate and gather new or changed variables again.' 
    addRemoveValue = Nothing 
    jobTitle = Nothing 
    jtLocation = Nothing 
    jtDepartment = Nothing 
    loc = Nothing 
    lComments = Nothing 
    department = Nothing 
    fname = Nothing 
    lname = Nothing 
    uID = Nothing 

    'Gather variables.' 
    addRemoveValue = TryCast(addRemoveComboBox.SelectedItem, String) 
    jobTitle = TryCast(jobTitleTextBox.Text, String) 
    jtLocation = TryCast(jtLocationComboBox.Text, String) 
    jtDepartment = TryCast(jtDepartmentComboBox.Text, String) 
    loc = TryCast(locationTextBox.Text, String) 
    lComments = TryCast(lCommentsTextBox.Text, String) 
    department = TryCast(departmentTextBox.Text, String) 
    fname = TryCast(firstNameTextBox.Text, String) 
    lname = TryCast(lastNameTextBox.Text, String) 
    uID = TryCast(uniqueIDTextBox.Text, String) 

    'I added this in order to troubleshoot the search feature for the Users search.' 
    'If uID = Nothing Then' 
    ' uID = "%"' 
    'End If' 

    'Change the visible buttons in the event of a Search.' 
    disableButton.Visible = True 
    addButton.Visible = False 

    'Populate the Data Grid based on the item the user selects.' 
    If addRemoveValue = "Job Title" Then 
     With Me.DataGridView1 
      .AutoGenerateColumns = True 
      .DataSource = RoleTableAdapter.GetDataByTitle(jobTitle, jtLocation, jtDepartment) 
      .AutoSizeRowsMode = DataGridViewAutoSizeRowsMode.DisplayedCellsExceptHeaders 
      .AutoSizeColumnsMode = DataGridViewAutoSizeColumnMode.DisplayedCells 
      .BorderStyle = BorderStyle.Fixed3D 
      .EditMode = DataGridViewEditMode.EditOnEnter 
     End With 
    End If 

    If addRemoveValue = "Location" Then 
     With Me.DataGridView1 
      .AutoGenerateColumns = True 
      .DataSource = LocationTableAdapter.GetDataByLoc(loc) 
      .AutoSizeRowsMode = DataGridViewAutoSizeRowsMode.DisplayedCellsExceptHeaders 
      .AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.DisplayedCells 
      .BorderStyle = BorderStyle.Fixed3D 
      .EditMode = DataGridViewEditMode.EditOnEnter 
     End With 
    End If 

    If addRemoveValue = "Department" Then 
     With Me.DataGridView1 
      .AutoGenerateColumns = True 
      .DataSource = DepartmentTableAdapter.GetDataByDept(department) 
      .AutoSizeRowsMode = DataGridViewAutoSizeRowsMode.DisplayedCellsExceptHeaders 
      .AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.DisplayedCells 
      .BorderStyle = BorderStyle.Fixed3D 
      .EditMode = DataGridViewEditMode.EditOnEnter 
     End With 
    End If 

    If addRemoveValue = "User" Then 
     With Me.DataGridView1 
      .AutoGenerateColumns = True 
      .DataSource = UsersTableAdapter.GetDataByName(uID, fname, lname) 
      .AutoSizeRowsMode = DataGridViewAutoSizeRowsMode.DisplayedCellsExceptHeaders 
      .AutoSizeColumnsMode = DataGridViewAutoSizeColumnsMode.DisplayedCells 
      .BorderStyle = BorderStyle.Fixed3D 
      .EditMode = DataGridViewEditMode.EditOnEnter 
      .Columns("Unique ID").DisplayIndex = 0 
      .Columns("Last Name").DisplayIndex = 1 
      .Columns("First Name").DisplayIndex = 2 
      .Columns("Middle Name").DisplayIndex = 3 
     End With 
    End If 
End Sub 

每当我测试，我收到以下错误：

Failed to enable constraints. One or more rows contain values violating non-null, unique, or foreign-key constraints.

我看过的所有列的属性，它们都可以包含空值。我查看了并确保我的桌子上没有外键约束。为了解决这个问题，我很困惑，为了解决这个问题，我在这段代码中进行的其他三项搜索工作得很好，并且他们有非常相似的查询。

任何帮助你可以提供将是很棒！谢谢！！

Answer 1

是否存在firstname，lastname和/或userid可能为空的情况？

有时，如果更改数据库中的列长度但不更新数据集，则有时会弹出此错误消息。