我有一个类似这样的表
+--------------------------+--------+------+---------+
| | City | Year | Density |
+--------------------------+--------+------+---------+
| Project 1 | City A | 2008 | 500 |
+--------------------------+--------+------+---------+
| Project 2 | City B | 2012 | 800 |
+--------------------------+--------+------+---------+
| Project 3 | City C | 2012 | 400 |
+--------------------------+--------+------+---------+
| Project 4 | City A | 2008 | 600 |
+--------------------------+--------+------+---------+
| Project 5 | City C | 2013 | 700 |
+--------------------------+--------+------+---------+
| etc (c. 30,000 projects spread across 30 cities) |
+--------------------------+--------+------+---------+
（约30,000个项目分布在30个城市。）
我可以编写如下查询：
SELECT Year, AVG(`Density`) as Density FROM table where City=’A’ GROUP BY Year
哪个适用于一个城市。任何人都可以指出我正确的方向，我如何写一个单独的查询，计算每个城市的年平均值？我期待一个看起来像这样的结果表：
+------+--------+--------+--------+-------------+
| | City A | City B | City C | City D, etc |
+------+--------+--------+--------+-------------+
| 2005 | | | | |
+------+--------+--------+--------+-------------+
| 2006 | | | | |
+------+--------+--------+--------+-------------+
| 2008 | | | | |
+------+--------+--------+--------+-------------+
| 2009 | | | | |
+------+--------+--------+--------+-------------+
| 2010 | | | | |
+------+--------+--------+--------+-------------+
| etc | | | | |
+------+--------+--------+--------+-------------+
我曾尝试在where子句(where in (select distinct City))中使用子查询，但这并不像我预期的那样。
或者我只需要手动为30个城市中的每个城市单独划线？
我不是MySQL的专家，无法从概念上看到我需要做什么。如果有人能给我任何指示，我将非常感激。谢谢。
答案 0 :(得分：0)
您可以按多列进行分组：
SELECT city, year, AVG(density) AS density
FROM table
GROUP BY city, year
这将为每个城市/年份组合返回单独的行。要将城市作为列，您需要转动它。见MySQL pivot table