관재탑

1. 집계함수 COUNT는 전제 행의 개수를 세줍니다. SELECT COUNT(*) FROM tips COUNT(DISTINCT 칼럼명)를 써주면 해당 칼럼에서 중복되는 행은 빼주고 유니크한 행의 개수를 세줍니다. SELECT COUNT(DISTINCT day) FROM tips COUNT는 NULL값은 제외하게 개수를 세줍니다. SUM, AVG, MIN, MAX가 있다. 2. GROUP BY GROUP BY는 그룹별로 요약해줍니다. 주로 집계함수와 같이 사용합니다. SELECT와 GROUP BY에는 캍은 컬럼이 들어가야 됩니다. SELECT day, time, ROUND(AVG(total_bill)) FROM tips GROUP BY day, time ORDER BY ROUND(AVG(total_bil..

SQL/MySQL 이론 2022. 7. 26. 11:38

1. WHERE에서 AND, OR, IN 적용 WHERE에서 OR로 조건을 연결할 때 local = '마포구' OR '동대문구'로 쓰면 안된다. local = '마포구' OR local = '동대문구' 처럼 칼럼명을 모두 써줘야 한다. SELECT * FROM station WHERE local = '마포구' OR local = '동대문구' IN()은 OR로 연결 된다. SELECT * FROM station WHERE local IN ('마포구', '서대문구') AND로 연결 SELECT * FROM station WHERE lng >= 126.75 AND local = '마포구' 2. ORDER BY로 정렬 updated_at 칼럼 기준으로 내림차순으로 정렬, 날짜를 내림차순 정렬하면 최근이 맨 위로..

SQL/MySQL 이론 2022. 7. 26. 10:50

https://www.hackerrank.com/challenges/symmetric-pairs/problem?isFullScreen=true Symmetric Pairs | HackerRank Write a query to output all symmetric pairs in ascending order by the value of X. www.hackerrank.com 문제 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 suc..

SQL/MySQL 문제풀이 2022. 7. 20. 16:40

https://leetcode.com/problems/rising-temperature/ Rising Temperature - LeetCode Level up your coding skills and quickly land a job. This is the best place to expand your knowledge and get prepared for your next interview. leetcode.com 문제 Table: Weather +---------------+---------+ | Column Name | Type | +---------------+---------+ | id | int | | recordDate | date | | temperature | int | +--------..

SQL/MySQL 문제풀이 2022. 7. 20. 15:31

https://leetcode.com/problems/employees-earning-more-than-their-managers/ Employees Earning More Than Their Managers - LeetCode Level up your coding skills and quickly land a job. This is the best place to expand your knowledge and get prepared for your next interview. leetcode.com 문제 Table: Employee +-------------+---------+ | Column Name | Type | +-------------+---------+ | id | int | | name |..

SQL/MySQL 문제풀이 2022. 7. 20. 14:19

https://leetcode.com/problems/customers-who-never-order/submissions/ Customers Who Never Order - LeetCode Level up your coding skills and quickly land a job. This is the best place to expand your knowledge and get prepared for your next interview. leetcode.com 문제 table: Customers +-------------+---------+ | Column Name | Type | +-------------+---------+ | id | int | | name | varchar | +---------..

SQL/MySQL 문제풀이 2022. 7. 20. 13:31

https://leetcode.com/problems/reformat-department-table/ Reformat Department Table - LeetCode Level up your coding skills and quickly land a job. This is the best place to expand your knowledge and get prepared for your next interview. leetcode.com 문제 +-------------+---------+ | Column Name | Type | +-------------+---------+ | id | int | | revenue | int | | month | varchar | +-------------+-----..

SQL/MySQL 문제풀이 2022. 7. 20. 11:04

https://www.hackerrank.com/challenges/what-type-of-triangle/problem?isFullScreen=true Type of Triangle | HackerRank Query a triangle's type based on its side lengths. www.hackerrank.com 문제 Write a query identifying the type of each record in the TRIANGLES table using its three side lengths. Output one of the following statements for each record in the table: Equilateral: It's a triangle with sid..

SQL/MySQL 문제풀이 2022. 7. 18. 19:58