我很确定SQLServer不理解这样的日期文字。这对access有用，但不同的数据库需要不同的语法。您应该使用yyyy-mm-dd格式的文本文字：

"' AND added_date BETWEEN '2020-07-21 00:00:00' AND '2020-07-21 23:59:59'"

也就是说，我猜您将不得不在某个时候改变这些日期，所以您可能应该使用参数。您肯定应该为 it_code 价值，例如。

Dim sql = "SELECT sum(purchase_qty) FROM TB_STOCK WHERE it_code = @it_code AND added_date BETWEEN @added_date_start AND @added_date_end"
Dim command As New SqlCommand(sql, connection)

With command.Parameters
    .Add("@it_code", SqlDbType.VarChar, 50).Value = itCode
    .Add("@added_date_start", SqlDbType.DateTime).Value = myDate
    .Add("@added_date_end", SqlDbType.DateTime).Value = myDate.AddDays(1).AddSeconds(-1)
End With

我同意rodney ellis不使用 BETEWEEN 我也是。我会这样做：

Dim sql = "SELECT sum(purchase_qty) FROM TB_STOCK WHERE it_code = @it_code AND added_date >= @added_date_start AND added_date < @added_date_end"
Dim command As New SqlCommand(sql, connection)

With command.Parameters
    .Add("@it_code", SqlDbType.VarChar, 50).Value = itCode
    .Add("@added_date_start", SqlDbType.DateTime).Value = myDate
    .Add("@added_date_end", SqlDbType.DateTime).Value = myDate.AddDays(1)
End With

最后两个例子都假设 myDate 时间已经归零了。如果没有，你可以用 myDate.Date .

Dim strSql As String = "SELECT sum(purchase_qty) FROM TB_STOCK 
WHERE it_code='" & itCode & "' AND added_date BETWEEN '2020-07-20 00:00:00' AND '2020-07-21 23:59:59'"

另外，不要在-https://www.mssqltips.com/sqlservertutorial/9316/sql-server-between-dates-issue/