Average of grouped rows in SQL

Question

I have the next table called BALL:

Id ...... Color ........... Month  
1 ........ blue ............ October
2 ........ red ............. January  
3 ........ green ........... September  
4 ........ red ............. October  
5 ........ red ............. March  
6 ........ blue ............ March  
7 ........ red ............. March  

The query I want to do is: Show the average number of colors blue month by month.

So I want the next result:

October 0.5
January 0
September 0
March 0.33

I was thinking in something like:

SELECT BALL.Month, avg(BALL.Color) 
FROM BALL
WHERE BALL.Color = 'blue'
GROUP BY BALL.Month;

But It didn't work,

Answer 1

You may use this method

SELECT BALL.Month, avg(BALL.Color='Blue') 
FROM BALL    
GROUP BY BALL.Month;

Answer 2

从1月份分组的balls选择MONTH，ROUND（AVG（color ='blue'），2）

Answer 3

Try this query. It is little complicated. But works

select t1.month,ifnull(t2.occur/t1.tot,0) as percentage from 
(select month, count(*) as tot from ball group by month) as t1
left join 
(select month, count(*) as occur from ball where color="blue" group by month) as t2 
on t1.month=t2.month

http://sqlfiddle.com/#!9/4f1fc/1

Answer 4

select t1.month,t1.mon_cnt,t2.col_cnt from  (select month,count(month) mon_cnt     from tab1 group by month) t1  join  (select month,count(month) col_cnt from tab1     where color='blue' group by month) t2 on (t1.month=t2.month);