Select the max on a field

Hi all,

Suppose that I have a table like this:

att_1 | att_2 | att_3 | att_4
--------------------------------
1 | a | y | y1
2 | b | y | y2
3 | a | xx | y3
4 | c | zz | y4
5 | a | t | y5
6 | c | x | y6

I want obtain all row that have for each att_2
the max of att_1

some like

SELECT *
FROM MY_TABLE
GROUP BY att_2
HAVING att_1 = max ( id_user_log)

for obtain:

att_1 | att_2 | att_3 | att_4
--------------------------------
2 | b | y | y2
5 | a | t | y5
6 | c | x | y6

Ciao
Gaetano