immediate-food-delivery-i.sql

/*

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.

Write an SQL query to find the percentage of immediate orders in the table, 
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           | 5           | 2019-08-02 | 2019-08-02                  |
| 3           | 1           | 2019-08-11 | 2019-08-11                  |
| 4           | 3           | 2019-08-24 | 2019-08-26                  |
| 5           | 4           | 2019-08-21 | 2019-08-22                  |
| 6           | 2           | 2019-08-11 | 2019-08-13                  |
+-------------+-------------+------------+-----------------------------+
Result table:

+----------------------+
| immediate_percentage |
+----------------------+
| 33.33                |
+----------------------+

The orders with delivery id 2 and 3 are immediate while the others are 

*/

# V0
SELECT
Round(100 * (SUM(when order_date = customer_pref_delivery_date then 1 else 0 end) / COUNT(deliver_id)), 2) AS immediate_percentage
FROM
Delivery

# V1
# https://code.dennyzhang.com/immediate-food-delivery-i
select round(100*sum(case when order_date=customer_pref_delivery_date then 1 else 0 end)/count(1), 2) immediate_percentage 
from Delivery

# V2
# Time:  O(n)
# Space: O(1)
SELECT Round(100 * Sum(order_date = customer_pref_delivery_date) / Count(*), 2) 
       AS 
       immediate_percentage 
FROM   delivery;