sql - Alternative to case statement -

i need suggestions on writing sql query other using case statement in oracle.i have table named system_specs following data

customer_id     disk_space_allocated c001                    44g c002                    1300g c003                    1503g c004                    1780g 

i wrote following sql query using oracle case statement count of customer_id based on ranges disk_space_allocated

select    case      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1  )) <= 300      '1-300'      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1  )) <= 500      '300-500'      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1 )) <= 700      '500-700'      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1  )) <= 900      '700-900'      else '900+'    end      diskallocated,    count(*) number_of_customers  system_specs  group    case      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1 )) <= 300      '1-300'      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1 )) <= 500      '300-500'      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1 )) <= 700      '500-700'      when to_number(substr(disk_space_allocated,0,length(disk_space_allocated) -1 )) <= 900      '700-900'      else '900+'    end; 

can query written other form?

firstly logic wrong calculating <=700 , saying between 500-700, if <=700 default <=500 , <=300. can not call between 500-700. try below query

select   count(*) number_of_customers,b.var "less below value" system_specs a, (select 300 + (level-1)*200 var  dual connect level <= 4) b to_number(substr(a.disk_space_allocated,0,length(a.disk_space_allocated) -1 )) <= b.var group b.var order b.var 

final answer question of @alex

with ranges (       select case when level = 1 0 else 100 end + (level-1) * 200 low_value,         case when level = 5 99999999 else 100 + (level) * 200 end high_value       dual       connect level <= 5     )     select r.low_value ||'-'|| r.high_value diskallocated,       count(*) number_of_customers     ranges r     left join system_specs ss     on to_number(substr(ss.disk_space_allocated, 1, length(ss.disk_space_allocated) -1 )) > r.low_value     , to_number(substr(ss.disk_space_allocated, 1, length(ss.disk_space_allocated) -1 )) <= r.high_value     group r.low_value, r.high_value     order r.low_value, r.high_value;