ImmediateFoodDelivery-II.sql

-- Table: Delivery

-- +-----------------------------+---------+
-- | Column Name                 | Type    |
-- +-----------------------------+---------+
-- | delivery_id                 | int     |
-- | customer_id                 | int     |
-- | order_date                  | date    |
-- | customer_pref_delivery_date | date    |
-- +-----------------------------+---------+
-- delivery_id is the column of unique values 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 customer's preferred delivery date is the same as the order date, then the order is called immediate; otherwise, 
-- it is called scheduled.

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

-- Write a solution to find the percentage of immediate orders in the first orders of all customers, rounded to 2 decimal places.

-- The result format is in the following example.

-- Example 1:

-- Input: 
-- 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                  |
-- +-------------+-------------+------------+-----------------------------+
-- Output: 
-- +----------------------+
-- | immediate_percentage |
-- +----------------------+
-- | 50.00                |
-- +----------------------+
-- Explanation: 
-- 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.


-- Write your PostgreSQL query statement below
-- Solution

with first_orders as 
(
    select customer_id,
        min(order_date) as first_order_date
    from Delivery group by customer_id
),
order_tags as 
(
    select delivery_id,
           customer_id,
           order_date,
           customer_pref_delivery_date,
           case when order_date = customer_pref_delivery_date 
           then 'immediate'
           else 'scheduled'
           end as order_type
    from Delivery
)
select round(count(ot.order_type)/(select count(*) from first_orders)::numeric, 4) * 100 
as immediate_percentage
from first_orders fo join order_tags ot
on (fo.customer_id = ot.customer_id 
    and fo.first_order_date = ot.order_date)
where ot.order_type = 'immediate';