SQL Query: Immediate Food Delivery solved using CTE

QUESTION: Table: Delivery

+-----------------------------+---------+
| Column Name                 | Type    |
+-----------------------------+---------+
| delivery_id                 | int     |
| customer_id                 | int     |
| order_date                  | date    |
| customer_pref_delivery_date | date    |
+-----------------------------+---------+
delivery_id is the primary key of this table.
The table holds information about food delivery to customers that make orders at some date and specify a preferred delivery date (on the same order date or after it).


If the preferred delivery date of the customer is the same as the order date then the order is called immediate otherwise it's called scheduled.

The first order of a customer is the order with the earliest order date that customer made. It is guaranteed that a customer has exactly one first order.

Write an SQL query to find the percentage of immediate orders in the first orders of all customers, rounded to 2 decimal places.

The query result format is in the following example:

Delivery table:
+-------------+-------------+------------+-----------------------------+
| delivery_id | customer_id | order_date | customer_pref_delivery_date |
+-------------+-------------+------------+-----------------------------+
| 1           | 1           | 2019-08-01 | 2019-08-02                  |
| 2           | 2           | 2019-08-02 | 2019-08-02                  |
| 3           | 1           | 2019-08-11 | 2019-08-12                  |
| 4           | 3           | 2019-08-24 | 2019-08-24                  |
| 5           | 3           | 2019-08-21 | 2019-08-22                  |
| 6           | 2           | 2019-08-11 | 2019-08-13                  |
| 7           | 4           | 2019-08-09 | 2019-08-09                  |
+-------------+-------------+------------+-----------------------------+

Result table:
+----------------------+
| immediate_percentage |
+----------------------+
| 50.00                |
+----------------------+
The customer id 1 has a first order with delivery id 1 and it is scheduled.
The customer id 2 has a first order with delivery id 2 and it is immediate.
The customer id 3 has a first order with delivery id 5 and it is scheduled.
The customer id 4 has a first order with delivery id 7 and it is immediate.
Hence, half the customers have immediate first orders.

SOLUTION:

Step1) Try to find first order of each customer using dense rank function
Step2) Get the immediate order from the first order set 
Step3) Get the percentage of immediate order to the total first order 

;with FirstOrderCTE as
(select 
 delivery_id, customer_id, order_date, 
 customer_pref_delivery_date, 
 dense_rank() over (partition by customer_id order by order_date asc) as rnk 
from Delivery
)
,immediate_order as
(select count(*) as immediate_order_count 
 from FirstOrderCTE 
 where rnk = 1 and order_date = customer_pref_delivery_date
)
 ,percent_of_immediate_order as
 (select 
  cast(((select immediate_order_count from immediate_order limit 1)/
  cast(count(*) as numeric(8,2))) * 100 as numeric(8,2)) as immediate_percentage,
  
  (select immediate_order_count from immediate_order limit 1),
  cast(count(*) as numeric(8,2))
  from FirstOrderCTE where rnk = 1
 )
 --select immediate_order_count from immediate_order limit 1
 select immediate_percentage from percent_of_immediate_order

Post Reference: Vikram Aristocratic Elfin Share