在mysql中使用group by子句查找平均值

Question

+--------------+  
| month | year |  
+-------+------+  
| jan   | 2015 |  
| jan   | 2015 |  
| jan   | 2015 |  
| feb   | 2015 |  
| feb   | 2015 |  
| feb   | 2015 |  
| feb   | 2015 |  
| feb   | 2015 |  
| mar   | 2015 |  
| mar   | 2015 |  
| mar   | 2015 |  
| mar   | 2015 |  
| mar   | 2015 |  
| mar   | 2015 |  
| mar   | 2015 |  
| mar   | 2016 |  
| mar   | 2016 |  
| mar   | 2016 |  
| mar   | 2016 |  
| mar   | 2016 |  
| mar   | 2016 |  
| mar   | 2016 |  
| mar   | 2016 |  
| feb   | 2016 |  
| feb   | 2016 |  
| jan   | 2016 |  
| jan   | 2016 |  
| jan   | 2016 |  
| jan   | 2016 |  
+--------------+

这是桌子。

我需要每个月的平均费率（即月/年）

Answer 1

您应该计算month列而不是* 。

这是查询：

SELECT (COUNT(my.`month`)/(SELECT COUNT(*) FROM crate inmn WHERE inmn.`year` = my.`year`)) AS num, my.`month`, my.`year` FROM crate my GROUP BY my.`month`,my.`year;

以上查询将给出特定year的平均month 。