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一步一个脚印，一天一道大数据面试题

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行转列

假设我们有一张名为 sales_data 的表，其中包含 product_id（产品 ID）、category（类别）和 sales_amount（销售金额）这几列的数据。

步骤：

1. group by id
2. 聚合函数sum/max/min，里面套一个 if / case when
3. as 列名

样例数据：

-- 样例 SQL
SELECT * FROM students;
+-----------+------------+-------------+
| stu_id    | subject    | score       |
+-----------+------------+-------------+
| 1         | Chinese    | 80          |
| 1         | Math       | 70          |
| 1         | English    | 75          |
| 2         | Chinese    | 77          |
| 2         | Math       | 60          |
| 2         | English    | 80          |
+-----------+------------+-------------+

开始行转列：

SELECT stu_id,SUM(IF(subject = 'Chinese', score, 0) AS chinese_score),SUM(IF(subject = 'Math', score, 0) AS math_score),SUM(IF(subject = 'English', score, 0) AS english_score)
FROM students
GROUP BY stu_id;+-----------+--------------+-------------+---------------+
| stu_id    | chinese_score| math_score  | english_score |
+-----------+--------------+-------------+---------------+
| 1         | 80           | 70          | 75            |
| 2         | 77           | 60          | 80            |
+-----------+--------------+-------------+---------------+

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