Protected Sub updatedata()

        Dim command As New MySqlCommand("UPDATE company_alert, company_financial SET company_alert.alert_value = (company_financial.receivable * 360 / company_financial.revenue)WHERE company_alert.alert_id='DSO' AND company_alert.stock_code = company_financial.stock_code AND company_alert.year_id = company_financial.year_id AND company_alert.period_id = company_financial.period_id", conn)

    End Sub
    Protected Sub insertdata()

        Dim strSQL As String
        strSQL = "INSERT INTO company_alert (stock_code, year_id, period_id,alert_id, alert_value) "
        strSQL = strSQL & "SELECT a.stock_code, a.year_id, a.period_id,alert_id,(a.receivable*360/a.revenue) AS dso1 "
        strSQL = strSQL & "FROM ref_alert_parameter, company_financial a "
        strSQL = strSQL & "WHERE alert_id='DSO' AND a.year_id BETWEEN '" & byear1 & "' AND '" & byear2 & "' AND a.period_id = '5'"

        cmd = New MySqlCommand(strSQL, conn)

        cmd.CommandTimeout = 0

    End Sub

How am i to do the checking in db that if stock_code already have in the company_alert it will go to insert..

You can check to see if certain data exists in db by using IF EXISTS(Condition) ELSE

IF EXISTS (SELECT * FROM company_alert WHERE stock_code = "your stock code")
 UPDATE company_alert, company_financial .
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