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  |-USER------|
  | id        | (a unique identifier for the row)
...
  | last_time | (the last time we saw the user)
  | url       | (the user's URL)
  |-----------|

  |-ALLIP-----|
  | id        | (a unique identifier for the row)
...
  | ip        | (the IP address)
  | time      | (the time it was seen)
  |-----------|

my $sql = <<SQLCODE;
  SELECT
...
{
    die( "Database error " . $dbh->errstr . "\n" );
}

my $found_id = undef;
my $sql = <<SQLCODE
...
    # perform insert
    # code left to read as an exercise
}