[Udemy] The Magic of Aggregate Functions

The Ultimate MySQL Bootcamp 9강을 공부하며 정리한 내용입니다.

COUNT

데이터의 개수를 계산한다.

SELECT COUNT(author_fname) FROM books; //19

//How many distinct author fnames?
SELECT COUNT(DISTINCT author_fname) FROM books; //12
//How many distinct author names?
SELECT COUNT(DISTINCT author_fname, author_lname) FROM books; 
//How many titles contain "the"?
SELECT count(title) FROM books WHERE title LIKE '%the%'; //6

GROUP BY

GROUP BY summarizes or aggregates identical data into single rows

SELECT title, author_lname 
FROM books 
GROUP BY author_lnames

위와 같은 테이블이 있고 다음과 같은 쿼리문을 작성했을때, 결과는 다음과 같다.

MIN, MAX

데이터의 최소값, 최대값을 찾는다.

SELECT MIN(released_year) FROM books; //1945
SELECT MAX(released_year) FROM books; //2017

• What if I want the title of the longest OR shortest book?
  SELECT MAX(pages), title FROM books;
  only_full_group_by 에러가 나거나 잘못된 제목이 조회된다. 즉, MAX(pages)는 올바르게 찾지만, 다음으로 찾아야하는 title은 책들 중 첫번째 제목을 조회한다.

  • 해결방안1 SUBQUERY
    단, 서브쿼리가 복잡해질수록 시간이 오래 걸린다.

    SELECT title, pages FROM books 
    WHERE pages = (SELECT MAX(pages) FROM books);

    
  • 해결방안2 ORDER BY, LIMIT
    SUBQUEYRY를 사용하지 않고 MAX,MIN의 특징을 토대로 문제를 해결할 수 있다.

    SELECT * FROM books ORDER BY pages DESC LIMIT 1

MIN, MAX with GROUP BY

• Find the year each author published their first book

SELECT author_fname, author_lname, MIN(released_year)
FROM books
GROUP BY author_lname, author_fname;

• Find the longest page count for each author

SELECT MAX(pages), author_lname, author_fname
FROM books
GROUP BY author_lname, author_fname;

SELECT (author_fname, ' ', author_lname) AS 'author', MAX(pages) AS 'longest book' 
FROM books
GROUP BY author_lname, author_fname; 

SUM

데이터의 합계를 구한다.

• Sum all pages each author has written

SELECT author_fname, author_lname, SUM(pages)
FROM books
GROUP BY author author_lname, author_fname;

AVG

데이터의 평균을 구한다.

• Calculate the average stock quantity for books released in the same year

SELECT AVG(stock_quantity), released_year 
FROM books
GROUP BY released_year;

• Calculate the average pages for for books written by the same author

SELECT author_lname, author_fname, AVG(pages)
FROM books
GROUP BY author_lname, author_fname;