1) when u write this way..
select count(distinct ACODE) from dbo.Table_1
then it counts for different values like my table(dbo.table_1) having ACODE column which has certain values
now if i execute first query will get ans = 3, bcoz it counts only distinct values.
select distinct count(ACODE) from dbo.Table_1
if u execute this then u will get answer = 8
bcoz, it has 3 times -2010, 3 times - 2011 , and 2 times - 2009
this count for all the records and give output, and does not give count for distinct value.
3)if u want to analyse it more deeply then try it by exicuting this query..
select distinct count(ACODE),ACODE from dbo.Table_1 group by ACODE
hope this help..