分组数据

SQL 中级

Jasmin Ludolf

Data Science Content Developer, DataCamp

分组数据

插图：一个满的漏斗将不同形状分流到三类：方形、圆形、三角形

按单字段 GROUP BY

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

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

错误处理

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        |
...

按多字段 GROUP BY

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 与 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       |
...

执行顺序

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

-- 执行顺序：

SELECT 
       certification, 
       COUNT(title) AS title_count

FROM films

GROUP BY certification

ORDER BY title_count DESC

LIMIT 3;

Passons à la pratique !

SQL 中级