将电影评分与客户数据关联

用 SQL 进行数据驱动决策

Irene Ortner

Data Scientist at Applied Statistics

JOIN

关于在 SQL 中连接数据的先修课程

LEFT JOIN

LEFT JOIN 属于外连接。
保留左表所有行，与右表匹配行对齐。
用标识符定义两表可匹配的行。

为表指定别名

SELECT *
FROM customers AS c
WHERE c.customer_id = 1;

LEFT JOIN 的表

左表：renting_selected

| renting_id | customer_id | rating |
|------------|-------------|--------|
| 518        | 1           | `null` |
| 203        | 2           | 6      |
| 478        | 4           | 7      |
| 292        | 4           | 8      |
| 477        | 5           | `null` |
| 400        | 6           | `null` |

右表：customers_selected

| customer_id | name               | gender |
|-------------|--------------------|--------|
| 1           | Robert Bohm        | male   |
| 2           | Wolfgang Ackermann | male   |
| 3           | Daniela Herzog     | female |
| 4           | Julia Jung         | female |

LEFT JOIN 示例

SELECT *
FROM renting_selected AS r
LEFT JOIN customers_selected AS c
ON r.customer_id = c.customer_id;

| renting_id | customer_id | rating | customer_id | name               | gender |
|------------|-------------|--------|-------------|--------------------|--------|
| 518        | 1           | `null` | 1           | Robert Bohm        | male   |
| 203        | 2           | 6      | 2           | Wolfgang Ackermann | male   |
| 478        | 4           | `null` | 4           | Julia Jung         | female |
| 292        | 4           | 8      | 4           | Julia Jung         | female |
| 477        | 5           | 7      | `null`      | `null`             | `null` |

多个 JOIN

SELECT m.title, 
       c.name
FROM renting AS r
LEFT JOIN movies AS m
ON r.movie_id = m.movie_id
LEFT JOIN customers AS c
ON r.customer_id = c.customer_id;

Ayo berlatih!

用 SQL 进行数据驱动决策