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 |
+----------+----------+

Here is a way to do it in SQL, for one customer.

select
   entity_id,
   increment_id,
   customer_is_guest
from
   sales_flat_order
where
   increment_id = 101125260
;

Outputs:

+-----------+--------------+-------------------+
| entity_id | increment_id | customer_is_guest |
+-----------+--------------+-------------------+
|   2371223 | 101125260    |                 1 |
+-----------+--------------+-------------------+

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 |
+----------+----------+