再加一个就行了 cast :

select cast(collect(
   cast(
     case
        when (1 = 1) then 
           'aaa'
        else 'bbb'
     end
     as varchar2(100)
    )
order by mt.my_col) as t_varchar2_tab)
from my_table mt;

完整示例：

with my_table as (select to_char(level,'000') my_col from dual connect by level<=10)
select cast(collect(
   cast(
     case
        when (1 = 1) then 
           'aaa'
        else 'bbb'
     end
     as varchar2(100)
    )
order by mt.my_col) as t_varchar2_tab)
 as res
from my_table mt;

RES
-------------------------------------------------------------------------------------
T_VARCHAR2_TAB('aaa', 'aaa', 'aaa', 'aaa', 'aaa', 'aaa', 'aaa', 'aaa', 'aaa', 'aaa')

另外，不要将eversql用于oraclesql。它只支持mysql