informix的sql语句中group by 后面不能跟case when 吗 ?

[informix_2015]

select
a1,
b1,
c1 ,
case when a in (1,2,3) then 'A'
when a in (4,5,6) then 'B'
when when a in (7) then 'C'
else 'D'
end a,
sum (d1),
sum(d2)
from tb_1
where e1=2
group by a1,
b1,
c1,
case when a in (1,2,3) then 'A'
when a in (4,5,6) then 'B'
when a in (7) then 'C'
else 'D'
end
;

[九月茅桃]

前面可以，后面不可以的。