VB.NET MYSQL Datagridview是空的吗？

Question

当我单击搜索按钮时，下面的代码将运行。 代码上没有错误，但datagridview仅显示列名。.查询是否有错误？

    mySqlConn = New MySqlConnection
    myCommand = New MySqlCommand
    dt = New DataTable()
    Try
        If dt IsNot Nothing Then
            dt.Dispose()
        End If
        If da IsNot Nothing Then
            da.Dispose()
        End If
        If DataGridView1.DataSource IsNot Nothing Then
            DataGridView1.DataSource = Nothing
        End If
        mySqlConn.ConnectionString = connStr
        myCommand.CommandText = "Select * from createproject Where (FloatNumber = @floatNo OR @floatNo is Null) AND (DeveloperName = @devName OR @devName is Null) AND (DevelopmentType = @devType OR @devType is Null) AND (LotPt = @lotPt OR @lotPt is Null) AND (Mukim = @mukim OR @mukim is Null) AND (Daerah = @daerah OR @daerah is Null) AND (Negeri = @negeri OR @negeri is Null) AND (TempReference = @tempRef OR @tempRef is Null)"
        myCommand.Connection = mySqlConn
        mySqlConn.Open()

        myCommand.Parameters.AddWithValue("@floatNo", TextBox3.Text.Trim())
        myCommand.Parameters.AddWithValue("@devName", ComboBox6.Text.Trim())
        myCommand.Parameters.AddWithValue("@devType", ComboBox7.Text.Trim())
        myCommand.Parameters.AddWithValue("@lotPt", TextBox4.Text.Trim())
        myCommand.Parameters.AddWithValue("@mukim", ComboBox8.Text.Trim())
        myCommand.Parameters.AddWithValue("@daerah", ComboBox9.Text.Trim())
        myCommand.Parameters.AddWithValue("@negeri", ComboBox10.Text.Trim())
        myCommand.Parameters.AddWithValue("@tempRef", TextBox6.Text.Trim())
        da = New MySqlDataAdapter(myCommand.ToString(), mySqlConn)
        da.SelectCommand = myCommand
        da.Fill(dt)
        DataGridView1.DataSource = dt
        da.Update(dt)
    Catch ex As MySqlException
        MsgBox(ex.ToString())
    Finally
        mySqlConn.Close()
        mySqlConn.Dispose()

    End Try

这是我点击搜索后

Answer 1

如果您打算在用户将字段保留为空的情况下忽略标准，那么在这种情况下，您实际上必须将NULL值传递给查询。 就像VB中的String.Empty和Nothing不一样，SQL中的空字符串和NULL也不一样。 您将必须执行以下操作：

Dim sql = <sql>
              SELECT *
              FROM MyTable
              WHERE (@Column1 IS NULL OR Column1 = @Column1)
              AND (@Column2 IS NULL OR Column2 = @Column2)
          </sql>

myCommand.CommandText = sql.Value

Dim column1 = TextBox1.Text.Trim()
Dim column2 = TextBox2.Text.Trim()

With myCommand.Parameters
    .Add("@Column1", SqlDbType.VarChar).Value = If(column1 = String.Empty, CObj(DBNull.Value), column1)
    .Add("@Column2", SqlDbType.VarChar).Value = If(column2 = String.Empty, CObj(DBNull.Value), column2)
End With

请注意，使用Add而不是AddWithValue来Add参数，因为无法从DBNull.Value推断数据类型。