Cách nhóm dữ liệu

SQL nâng cao

Jasmin Ludolf

Data Science Content Developer, DataCamp

Cách nhóm dữ liệu

Hình minh họa thể hiện một phễu đầy đủ với các hình dạng khác nhau được phân loại vào ba nhóm: hình vuông, hình tròn và hình tam giác

GROUP BY các trường đơn lẻ

SELECT certification, COUNT(title) AS title_count
FROM films
GROUP BY certification;

|certification|title_count|
|-------------|-----------|
|Unrated      |62         |
|M            |5          |
|G            |112        |
|NC-17        |7          |
...

Xử lý lỗi

SELECT certification, title
FROM films
GROUP BY certification;

column "films.title" must appear in the
GROUP BY clause or be used in an 
aggregate function
LINE 1: SELECT certification, title
                              ^

SELECT 
   certification, 
   COUNT(title) AS count_title
FROM films
GROUP BY certification;

|certification|count_title|
|-------------|-----------|
|Unrated      |62         |
|M            |5          |
|G            |112        |
...

GROUP BY nhiều trường

SELECT certification, language, COUNT(title) AS title_count
FROM films
GROUP BY certification, language;

|certification|language |title_count|
|-------------|---------|-----------|
|null         |null     |5          |
|Unrated      |Japanese |2          |
|R            |Norwegian|2          |
...

GROUP BY với ORDER BY

SELECT 
       certification, 
       COUNT(title) AS title_count
FROM films
GROUP BY certification;

|certification|title_count|
|-------------|-----------|
|Unrated      |62         |
|M            |5          |
|G            |112        |
...

SELECT 
       certification, 
       COUNT(title) AS title_count
FROM films
GROUP BY certification
ORDER BY title_count DESC;

|certification|title_count|
|-------------|-----------|
|R            |2118       |
|PG-13        |1462       |
...

Thứ tự thực thi

-- Written code:
SELECT 
       certification, 
       COUNT(title) AS title_count
FROM films
GROUP BY certification
ORDER BY title_count DESC
LIMIT 3;

-- Order of execution:

SELECT 
       certification, 
       COUNT(title) AS title_count

FROM films

GROUP BY certification

ORDER BY title_count DESC

LIMIT 3;

Cùng thực hành nào!

SQL nâng cao