DATASET QUERY DROP TABLE IF EXISTS mst_users_with_dates; CREATE TABLE mst_users_with_dates ( user_id varchar(255) , register_stamp varchar(255) , birth_date varchar(255) ); INSERT INTO mst_users_with_dates VALUES ('U001', '2020-02-28 10:00:00', '2000-02-29') , ('U002', '2020-02-29 10:00:00', '1992-03-29') , ('U003', '2020-03-01 10:00:00', '2002-01-29') , ('U004', '2020-03-01 11:00:00', '1988-08-..

선택도와 카디널리티 1. 선택도 : 조건절에 의해 선택되는 레코드 비율 선택도 = 1 / NDV(컬럼 값 종류 개수) 카디널리티 : 전체 레코드 중 조건절에 의해 선택되는 레코드 수 카디널리티 = 총 로우 수 * 선택도 = 총 로우 수 / NDV 옵티마이저는 카디널리티를 구하고, 데이터를 액세스하는데 드는 비용을 계산해서 데이블 액세스 방식, 조인순서, 조인방식 등을 결정 선택도를 제대로 계산하는 것이 중요 통계정보 테이블 통계 인덱스 통계 컬럼 통계 : 테이블 통계 수집할 때 함께 수집. '=' 조건에 대한 선택도는 데이터 분포가 균일하지 않은 컬럼에서는 잘못 산출될 수 있기 때문에 옵티마이저는 일반적인 컬럼통계 외에 히스토그램을 추가로 활용 시스템 통계 : 애플리케이션 및 하드웨어 성능 특성 측정 C..

www.hackerrank.com/challenges/15-days-of-learning-sql/problem 15 Days of Learning SQL | HackerRank find users who submitted a query every day. www.hackerrank.com Julia conducted a days of learning SQL contest. The start date of the contest was March 01, 2016 and the end date was March 15, 2016. Write a query to print total number of unique hackers who made at least submission each day (starting ..

Samantha interviews many candidates from different colleges using coding challenges and contests. Write a query to print the contest_id, hacker_id, name, and the sums of total_submissions, total_accepted_submissions, total_views, and total_unique_views for each contest sorted by contest_id. Exclude the contest from the result if all four sums are 0. Note: A specific contest can be used to screen..

www.hackerrank.com/challenges/symmetric-pairs/problem Symmetric Pairs | HackerRank Write a query to output all symmetric pairs in ascending order by the value of X. www.hackerrank.com You are given a table, Functions, containing two columns: X and Y. Two pairs (X1, Y1) and (X2, Y2) are said to be symmetric pairs if X1 = Y2 and X2 = Y1. Write a query to output all such symmetric pairs in ascendin..

www.hackerrank.com/challenges/placements/problem Placements | HackerRank Write a query to output the names of those students whose best friends got offered a higher salary than them. www.hackerrank.com You are given three tables: Students, Friends and Packages. Students contains two columns: ID and Name. Friends contains two columns: ID and Friend_ID (ID of the ONLY best friend). Packages contai..

www.hackerrank.com/challenges/sql-projects/problem SQL Project Planning | HackerRank Write a query to output the start and end dates of projects listed by the number of days it took to complete the project in ascending order. www.hackerrank.com You are given a table, Projects, containing three columns: Task_ID, Start_Date and End_Date. It is guaranteed that the difference between the End_Date an..

www.hackerrank.com/challenges/contest-leaderboard/problem Contest Leaderboard | HackerRank Generate the contest leaderboard. www.hackerrank.com You did such a great job helping Julia with her last coding contest challenge that she wants you to work on this one, too! The total score of a hacker is the sum of their maximum scores for all of the challenges. Write a query to print the hacker_id, nam..

www.hackerrank.com/challenges/challenges/problem Challenges | HackerRank Print the total number of challenges created by hackers. www.hackerrank.com Julia asked her students to create some coding challenges. Write a query to print the hacker_id, name, and the total number of challenges created by each student. Sort your results by the total number of challenges in descending order. If more than ..

www.hackerrank.com/challenges/harry-potter-and-wands/problem Ollivander's Inventory | HackerRank Help pick out Ron's new wand. www.hackerrank.com Harry Potter and his friends are at Ollivander's with Ron, finally replacing Charlie's old broken wand. Hermione decides the best way to choose is by determining the minimum number of gold galleons needed to buy each non-evil wand of high power and age..