R语言对数据框按照某个字段排序

a=data.frame(a1=c(2,1,3,4,2,5,4),a2=c(7,4,5,6,5,6,4),a3=c(1,2,3,4,5,6,7))
a
a1 a2 a3
1  2  7  1
2  1  4  2
3  3  5  3
4  4  6  4
5  2  5  5
6  5  6  6
7  4  4  7

a[order(a$a1,-a$a2),]
a1 a2 a3
2  1  4  2
1  2  7  1
5  2  5  5
3  3  5  3
4  4  6  4
7  4  4  7
6  5  6  6

按a1升序，a2降序（a1相同的基础上）排列