mysql - SQL - 将组元素转换为新列

Question

我手头得到以下查询结果:

SELECT DISTINCT director, name
FROM Movie m
JOIN Rating ra ON m.mid = ra.mid
JOIN Reviewer re ON ra.rid = re.rid
WHERE director IS NOT NULL
ORDER BY director, name;
+------------------+------------------+
| director         | name             |
+------------------+------------------+
| James Cameron    | Elizabeth Thomas |
| James Cameron    | James Cameron    |
| Robert Wise      | Brittany Harris  |
| Robert Wise      | Chris Jackson    |
| Steven Spielberg | Ashley White     |
| Steven Spielberg | Brittany Harris  |
| Steven Spielberg | Chris Jackson    |
| Victor Fleming   | Mike Anderson    |
| Victor Fleming   | Sarah Martinez   |
+------------------+------------------+

最终我想获得一个数据透视表，其中所有董事都在一行，所有匹配的审稿人姓名都在自己的列中。就像这样:

director         | name1            | name2           | name3
-----------------+------------------+-----------------+---------------+
James Cameron    | Elizabeth Thomas |James Cameron    | <null>
Steven Spielberg | Ashley White     | Brittany Harris | Chris Jackson
.......

任何帮助将不胜感激!

Answer 1

MySQL 没有内置的数据透视功能。另外，你怎么知道正好有3个审稿人呢？ SQL查询需要返回固定数量的列。

相反，您可以使用 group_concat() 将所有审阅者放在一个列表中:

SELECT m.director, GROUP_CONCAT(r.name SEPARATOR ', ') as reviewers
FROM Movie m JOIN
     Rating ra
     ON m.mid = ra.mid JOIN
     Reviewer re
     ON ra.rid = re.rid
WHERE director IS NOT NULL
GROUP BY m.director
ORDER BY director;

如果审阅者可以多次审阅一位导演，那么您需要DISTINCT:

SELECT m.director, GROUP_CONCAT(DISTINCT r.name SEPARATOR ', ') as reviewers