how to calculate expiration date VB.NET

Question

 Dim readers As MySqlDataReader

    cn.Open()
    Dim query As String
    query = "Select * from tblmeds where expdate='" & DateTime.Now & "'"
    command = New MySqlCommand(query, cn)
    readers = command.ExecuteReader

    Dim count As Integer
    count = 0
    While readers.Read
       count = count + 1
    End While

        cn.Close()

        If count = 0 Then

           MsgBox("no expiration")
        Else
            MsgBox("medicine at risk")
        End If

this is just the code i made but it doesnt work because it doesnt go to while instead is goes straight for if count=0. also how can i say to my program "if the expiration date on the database is near month i set to" sorry for my bad english

Answer 1

You are checking if any of your items has an expiry date exactly equals to the current value of DateTime.Now. This property contains also the Time part and thus it is highly improbable that you have an item with exactly the same value.

I think you would use instead 'expdate lesser than now' thus your code should be

Dim readers As MySqlDataReader
cn.Open()
Dim query As String
query = "Select * from tblmeds where expdate<=@exp"
command = New MySqlCommand(query, cn)
command.Parameters.Add("@exp", MySqlDbType.DateTime).Value = DateTime.Now;
readers = command.ExecuteReader
Dim count As Integer
count = 0
While readers.Read
   count = count + 1
End While
cn.Close()
If count = 0 Then
    MsgBox("no expiration")
Else
    MsgBox("medicine at risk")
End If

Instead if you want to check for expiry date equals to today date then the expression to use is DateTime.Today instead of DateTime.Now.

Notice also that is a very good practice to never concatenate strings to form sql queries. Use always a parameter approach to avoid parsing and conversion errors.

Answer 2

为什么不使用时间戳记差异：

query = "Select * from tblmeds where TIMESTAMPDIFF(MONTH,`expdate`,CURRENT_TIMESTAMP())< 1"