vb.net 如何从数据库加载当前月份或任何特定月份

使用 MONTH 函数

Dim cmd As New MySqlCommand(
    " select * from tblchecklist " & 
    " where MONTH(checkdate) = " & DateTime.Now.Month & 
    " and check_number = '" & Label1.Text & "'", conn)

我只是在回答你的问题。但你也应该

• 参数化查询
• 在后台线程上运行查询，而不是在 UI 线程上运行查询
• 处置 IDisposable 对象
• 为控件命名
• 不使用 ，而是明确选择您需要的内容SELECT *

这改善了一些事情：

Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    TextBox1.Text = DateTime.Now.Month.ToString
    Try 
         'Do NOT try to re-use the same connection object throughout your app or form
         'Thanks to connection pooling, it really is better to create (and immediately dispose)
         ' a new object for every query
         Dim cn As New MySqlConnection("connection string here")

         'Do NOT use string concatenation to put values into an SQL query!
         Dim cmd As New MySqlCommand("select * from tblchecklist where checkdate = @Month and checkNumber = @CheckNum", cn)

         cmd.Parameters.AddWithValue("@Month", DateTime.Now.Month)
         cmd.Parameters.AddWithValue("@CheckNum", Label1.Text)

         cn.Open()
         'Rather than looping through results, you can often assign the query result object as a data source
         DataGridView1.DataSource = cmd.ExecuteReader()
     Catch ex As Exception
         MsgBox(ex.Message)
     ' Use a Finally (or Using) block to close/dispose your connection object
     Finally
         cn.Dispose()
     End Try
 End Sub

但这仍然是错误的。该属性只是一个整数。听起来您希望查询显示整个月的任何内容。Month

在这种情况下，最佳选项如下所示：

Private Sub Form2_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    TextBox1.Text = DateTime.Now.Month.ToString
    Dim SQL As String = "
SELECT ID, check_corporate, check_date, check_num, check_name, check_memo,
    check_amount, check_group, date_deposit, date_release, status
FROM tblchecklist 
WHERE checkdate >= @MonthStart AND checkdate < DATE_ADD(@MonthStart, INTERVAL 1 MONTH) 
    AND checkNumber = @CheckNum"

    Try 
         Dim cn As New MySqlConnection("connection string here")
         Dim cmd As New MySqlCommand(SQL, cn)

         cmd.Parameters.AddWithValue("@MonthStart", New DateTime(DateTime.Today.Year, DateTime.Today.Month, 1))
         cmd.Parameters.AddWithValue("@CheckNum", Label1.Text)

         cn.Open()
         DataGridView1.DataSource = cmd.ExecuteReader()
     Catch ex As Exception
         MsgBox(ex.Message)
     Finally
         cn.Dispose()
     End Try
 End Sub

这里最大的变化是计算日期范围。这比检查每行中列的月份值要好得多，因为您只需要执行一次，而不是对表中的每个值执行一次，并且因为它仍然可以处理表中的索引。

我还想直接解决这个问题：

如果我的 mysqlcommand 中有 2 个或更多“where”语句，它还会运行吗？

不。你只得到一个子句。但是，该子句可以有任意数量的布尔条件（谓词）通过 和 运算符连接，并且只要用户提供的任何文本使用查询参数，就可以使用字符串连接来动态构建此子句。WHEREANDOR