BÀI Given the following purchases table that record all purchases of an online ecommerce store: create table purchases ( id integer primary key, user_id integer, product_id integer, created_at timestamp without timezone ) Please write a SQL query that show monthly unique buyers, broken down by first time buyers, and returned buyers A first time buyer is a buyer that hasn't made any purchase prior A returned buyer is a buyer that has made at least purchase prior Your SQL should return columns: - month (date) - first_time_buyers (integer), - returned_buyers (integer) Please submit your work (SQL) and state the SQL database you're using (MySQL, PostgreSQL, SQL Server or any other DBs) You can provide any explanation if you want to Examples (data below has been simplified for ease of understanding, not how it is actually stored in the table, e.g user_id will only contain integer) id | user_id | product_id | created_at | | alice | | Jul 2016 | | charlie | | Jul 2016 | | alice | | Aug 2016 | | bob | | Aug 2016 | | david | | Aug 2016 | | bob | | Sep 2016 | | bob | | Sep 2016 | | david | | Sep 2016 | Expected results: | month | first_time_buyers | returned_buyers | | 2016-07-01 | | | | 2016-08-01 | | | | 2016-09-01 | | | TEST DATA create database MyERP go use MyERP go create table purchases ( id integer primary key IDENTITY(1,1), user_id integer, product_id integer, created_at datetime ) insert into purchases values (2, 2, '2016-07-10'); insert into purchases values (9, 3, '2016-07-17'); insert into purchases values (1, 4, '2016-07-18'); insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert insert into into into into into into into into into into into into into into into into into into into into into into into into into into purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases purchases values values values values values values values values values values values values values values values values values values values values values values values values values values (1, (2, (1, (1, (5, (1, (1, (2, (1, (1, (4, (1, (5, (1, (1, (1, (2, (3, (1, (2, (1, (1, (1, (1, (1, (1, 2, 5, 4, 3, 2, 1, 2, 2, 3, 4, 2, 5, 4, 1, 3, 2, 2, 3, 4, 5, 4, 1, 3, 2, 2, 4, '2016-07-20'); '2016-07-21'); '2016-07-25'); '2016-07-25'); '2016-07-25'); '2016-07-25'); '2016-09-25'); '2016-09-10'); '2016-09-17'); '2016-09-18'); '2016-09-20'); '2016-09-21'); '2016-09-21'); '2016-09-21'); '2016-09-21'); '2016-08-20'); '2016-08-10'); '2016-08-17'); '2016-08-18'); '2016-08-21'); '2016-08-25'); '2016-08-25'); '2016-08-25'); '2016-08-25'); '2016-01-25'); '2017-01-10'); RESULT LIKE THIS YYYY-MM-DD 2016-01-01 2016-07-01 2016-08-01 2016-09-01 2017-01-01 FIRST-TIME_BUYERS 1 BÀI create database Hacker_rank use hacker_rank create table project ( task_id int primary key, start_date smalldatetime, end_date smalldatetime ) set dateformat ymd RETURNED_BUYERS insert insert insert insert insert insert insert into into into into into into into project project project project project project project values(1, values(2, values(3, values(4, values(5, values(6, values(7, '2015-10-01', '2015-10-02', '2015-10-03', '2015-10-13', '2015-10-14', '2015-10-28', '2015-10-30', '2015-10-02'); '2015-10-03'); '2015-10-04'); '2015-10-14'); '2015-10-15'); '2015-10-29'); '2015-10-31'); Bảng Project chứa task project Những task có ngày kết thúc ngày b đ ầu c task khác chúng thuộc project Hãy liệt kê project, thời lượng, ngày bắt đầu với kết xếp theo thời lượng tăng dần, ngày bắt đầu giảm dần