Here is a way to do it in SQL, to generate a histogram and understand the percentage of your users which are logged in when checking out.

    select
       if(customer_id, 1, 0) as loggedin,
       count(*)
    from
       sales_flat_order
    where
       created_at > '2018-11-01'
    group by
       1
    order by
       1 desc
    ;

Outputs:

    +----------+----------+
    | loggedin | count(*) |
    +----------+----------+
    |        1 |     1194 |
    |        0 |     1693 |
    +----------+----------+