filter alarm table

[Frank68]

Hello

I should only select alarms from the previous month and then export them to a csv file and send the whole thing via email.
But I thought about the query, I'm using the following code.

Code:

--My (smtps) username !IMPORTANT!
user = 'report@test.com'
--My (smtps) password !IMPORTANT!
password = 'xxxxxx'

--Sender for e-mail
from = '<' .. user .. '>'
alias_from = 'Do not Reply '

--Recipient for e-mail
to = '<xxxx@xxxx.com>'
alias_to = to

--Subject for e-mail
subjectpart1 = 'Lista Allarmi Mese'
subjectpart2 = 'E-Mail inviata in automatico dal WebServer'

--Message on bottom of email (will only be showed when client don't understand attachment)
epilogue = 'End of message'

-- vedere come filtrare mese se 1 prendere 12 e anno -1
----------------------------------------------------------------
if (os.date('%m')==1) then
    mp=12
    yr=os.date('%Y')-1
else 
    mp=os.date('%m')-1
    yr=os.date('%Y')
end
------------------------------------------------------------------

--log('MESE ' ..mp .. ' ANNO ' ..yr)

-- Get all alerts from DB
alerts_table = db:getall('SELECT * FROM alerts where alerttime like %'..mp..'.'..yr..'% ORDER BY id DESC')
-- csv buffer
buffer = {}
-- format csv row
csv = string.format('%q,%q,%q', "ID", "Messaggio", "Data e Ora")
-- add to buffer
table.insert(buffer, csv)
-- add empty line to buffer
table.insert(buffer, "")

-- Loop through alerts_table
for _, alerts in ipairs(alerts_table) do
  -- format csv row
  csv = string.format('%q,%q,%q', alerts.id, alerts.alert, os.date("%x %X", alerts.alerttime))
  -- add to buffer
  table.insert(buffer, csv)
end

     --Create table to include mail settings
    local settings = {
        from = from,
        rcpt = to,
        user = user,
        password = password,
        server = 'smtps.aruba.it',
        port = 465,
        secure = 'sslv23',
    }

    --Create attachment inside FTP server
    src = os.date('%Y-%m') .. '.csv'
    dst = '/home/ftp/' .. src
    io.writefile(dst, buffer)

    --Create subject
    subject = subjectpart1 .. ": " .. src .. " " .. subjectpart2

    --Load required modules to send email with attachment
    local smtp = require("socket.smtp")
    local mime = require("mime")
    local ltn12 = require("ltn12")

    --Create e-mail header
    settings.source = smtp.message{
    headers = {
              from = '' .. alias_from .. ' ' .. from .. '',
              to = '' .. alias_to .. ' ' .. to .. '',
              subject = subject
    },

    --Load attachment inside body     
    body = {
    preamble = "",
     [1] = { 
            headers = {
               ["content-type"] = 'text/plain',
               ["content-disposition"] = 'attachment; filename="'..src..'"',
               ["content-description"] = '.. src ..',
               ["content-transfer-encoding"] = "BASE64",
            },
            body = ltn12.source.chain(
              ltn12.source.file(io.open(dst, "rb")),
              ltn12.filter.chain(
              mime.encode("base64"),
              mime.wrap()
            )
          )
        },
          epilogue = epilogue
      }
    }

    --Send the email
    r, e = smtp.send(settings)

    --Create alert when sending gives an error with error message
    if (e) then
      log (e)
      log (r)
      alert("Could not send email: ", e, "\n")
    end

  --Delete created file from ftp folder inside HL
  os.remove(dst)

taking into account that if month = 1 I have to take the data of month 12 and previous year
my problem is the select query I don't know how to filter the column data whose name I don't know (alarm date)

grazie