Putting unique values in a database

I was implementing the POP3 protocol in python for my Project Lab and I’d like to put some interesting tidbits I learned in the process.

A small problem was, whenever you query the mail server using the LIST command, it will always return all the messages which are there in the inbox, so its our job to check for unique mails and to store only those. A wrote a simple python script to do just that. Now my msg_id column has an index defined for it. and I’m hashing all the message id’s and storing them in the database for uniformity.

conn = sql.connect("mail")
c = conn.cursor()
m = hashlib.md5()
m.update(msgid)
c.execute('select exists(select * from mail where msg_id=?)',[m.hexdigest()])
for row in c:
    predict = row[0]
if not int(predict) :
    c.execute("insert into mail values(:body, :date, :msgid, :p_from, :sub)",
            {"body":body,"date":date,"msgid":m.hexdigest(),"p_from":p_from, "sub":sub})