SQL relational database: Data manipulation and calculations

Question

I have two tables. One is a Booking_Platform table with all hotel booking data. The second is a Customer_Country_Table which stores information for origins of each customer that booked hotels through the platform.

I have to calculate which country has the highest increase in bookings from 2017 to 2018.

I will give some sample data below for reference:

Booking_Platform_Info
Booking_Date    column2  column3....  column N .......  Origin_Country_ID 
20-dec-2016    ....................................          103
03-jan-2017    ....................................          101
09-feb-2017    ....................................          103
23-apr-2017    ....................................          102
06-oct-2017    ....................................          102
11-nov-2017    ....................................          103
05-jan-2018    ....................................          102
21-jan-2018    ....................................          102
26-feb-2018    ....................................          101
09-mar-2018    ....................................          101
11-may-2018    ....................................          103
10-sep-2018    ....................................          102
20-nov-2018    ....................................          101
07-dec-2018    ....................................          101
23-dec-2018    ....................................          101
31-dec-2018    ....................................          103
23-jan-2019    ....................................          103


Customer_Country_Info
Country_ID        Country_Name
101                  Italy
102                  Spain
103                  Portugal

It is a bit complicated for me, as I understand I have to first join the tables, then do a group by country, then count the total no. of bookings by year (probably another group by); and then compare the results to see which country has the highest positive difference in bookings from 2017-2018. I welcome any help with coding this problem.

In my example, country 101 Italy would be the answer because difference between bookings in 2018 and 2017 is highest (5-1=4)

*********Edit after comments

I am writing two queries to get booking totals by country ID for both 2017 and 2018

 SELECT CAST(booking_date AS DATE), COUNT(*) as number_of_bookings, origin_country_id FROM Booking_Platform_Info                
WHERE booking_date >= '2017-01-01' AND              
      booking_date < '2017-01-01'               
GROUP BY origin_country_id;             

 SELECT CAST(booking_date AS DATE), COUNT(*) as number_of_bookings, origin_country_id FROM Booking_Platform_Info                
WHERE booking_date >= '2018-01-01' AND              
      booking_date < '2019-01-01'               
GROUP BY origin_country_id;

Sorry for my lack of knowledge, but I am not aware how to join queries so that I could get the country id with the highest difference in bookings.

Answer 1

You need to join those two queries to compare the counts.

You also shouldn't include CAST(booking_date AS DATE) in the SELECT list. It's not needed, and it will just be a randomly selected date from the year.

SELECT country_name
FROM (
    SELECT a.origin_country_id
    FROM (
        SELECT origin_country_id, COUNT(*) AS 2017_total
        FROM Booking_Platform_Info
        WHERE STR_TO_DATE(booking_date, '%d-%b-%Y')  BETWEEN '2017-01-01' AND '2017-12-31'
    ) AS a
    JOIN (
        SELECT origin_country_id, COUNT(*) AS 2018_total
        FROM Booking_Platform_Info
        WHERE STR_TO_DATE(booking_date, '%d-%b-%Y') BETWEEN '2018-01-01' AND '2018-12-31'
    ) AS b
    ORDER BY 2018_total - 2017_total
    LIMIT 1
) as t1
JOIN Customer_Country_Info AS t2 ON t1.origin_country_id = t2.origin_country_id