Diademata

1.단일 책임 원칙(SRP, Single Responsibility Principle) 클래스를 변경해야 할 이유는 단 한가지여야 한다. 이 원칙은 클래스 응집도와 관련이 있다. 책임이 하나뿐인 응집도 높은 클래스를 만든다면 이 원칙을 충족시키는 것이다. 2.개방 폐쇄 원칙(OCP, Open-Closed Principle) 소프트웨어의 구성요소는 확장에 관해서는 열려있어야 하고 변경에 대해서는 닫혀있어야 한다. 클래스 상속을 생각해보면 부모 클래스가 닫힌 부분이고 자식 클래스는 열린 부분이다. 부모 클래스를 굳이 변경하지 않더라도, 자식 클래스의 행동을 변화시키면 확장할 수 있다. 3.리스코프 치환 원칙(LSB, Liskov Substitution Principle) 파생 자료형은 기본 자료형과 치환 할..

Your company hired an intern database engineer, who immediately started updating the data in the system. Unfortunately, he hasn't fully grasped the concept of NULL values yet and he performed some incorrect inserts and updates to the departments table, which has the following structure: id: the unique department ID; name: the name of the department; description: the description of the department..

한 컬럼의 여러 값을 조회 => Index 값이 나온다. name hobbies Adam swimming Amy reading,sports,swimming Carl filmwatching,writing Carol reading,swimming Deril sports Jake reading,sports,swimming Lola reading,sports,painting Nina sports,painting Sam sports select name, FIND_IN_SET('reading', hobbies) as idx from people_hobbies; name idx Adam 0 Amy 1 Carl 0 Carol 1 Deril 0 Jake 1 Lola 1 Nina 0 Sam 0

id characters color 1 code blue 2 fights white SELECT EXP(SUM(LOG(LENGTH(characters)))) as combinations from discs; EXP SUM LOG 로 계산한다.

MySQL Troubleshooting 발췌 MySql 서버는 언제나 입력한 대로 쿼리를 실행하지 않는다. 대신 사용자가 더 빨리 결과를 얻을 수 있도록 쿼리 옵티마이저를 호출하여 더 좋은 실행계획을 만든다. 테이블이 Index를 가지고 있지 않는 경우 MySql는 결과를 제한할 수 있을때 인덱스를 사용한다. 따라서 Where, JOIN, Group By, Order by 절에 들어가는 열이 인덱스를 가지고 있다면 성능이 향상된다. ※ EXPLAIN EXTENDED SELECT 구문을 사용한 후 SHOW WARNINGS 을 사용하면 최적화된 쿼리를 보여준다. id select_type table partitions type possible_keys key key_len ref rows filtered E..

Implement the missing code, denoted by ellipses. You may not modify the pre-existing code.You've been dreaming about becoming a famous hacker all your life, and now it's time to make your dreams come true! You decided to start by finding a website that has some vulnerability, and you just found a doozy. This particular website has an open database users that contains information about the people..

Your company is planning to expand internationally very soon. You have been tasked with preparing a report on foreign markets and potential competitors. After some investigation, you've created a database containing a foreignCompetitors table, which has the following structure: competitor: the name of the competitor;country: the country in which the competitor is operating.In your report, you ne..

You have a table soccer_team that contains information about the players in your favorite soccer team. This table has the following structure: id: the unique ID of the player;first_name: the first name of the player;surname: the last name of the player;player_number: the number that the player wears (the number is guaranteed to be unique).Create a semicolon-separated list of all the players, sor..

You are an avid traveler and you've visited so many countries that when people ask you where you've been, you can't even remember all of them! Luckily, every time you travel somewhere you write down the trip information in your diary. Now you want to get a list of all the different countries that you have visited using the information in your diary. The diary is represented as a table diary, whi..

You want to expand your movie collection, but you don't really have any preferences so you're not sure where to start. After some consideration, you decide that you should start by finding more movies from award-winning directors whose movies you already own and who have shot a movie somewhat recently. To find the directors whose movies you might want to consider watching in the first place, you..