Suppose I have such a table : 

```
theme    module  changed
0        1       1426070691
1        1       1426070300
0        1       1324014321
1        2       1241245585
1        1       1015421251

```

I need a SQL query that returns for the same theme and module the max value of changed:

```
theme    module  changed
0        1       1426070691
1        1       1426070300
1        2       1241245585

```