How to find percentage in SQL from list of zeros and ones?

I have result set like -

id achieved
1    0
2    1
3    1
4    0 
5    0

The Percentage should be 2/5 i.e. 40 %. How can I write a SQL Query to achieve something 开发者_Go百科like this ? I would prefer not to use and nested select as the actual query is already doing quite a bit. Thanks !

select avg(achieved) from ... 

Note that you will have to use a group by function to include categories:

select gender, avg(achieved) from ... group by gender