select seq,
       count(1) n
  from (
select case when t.num>10 and t.num<=15 then 'a_10_15'
            when t.num>15 and t.num<=30 then 'a_15_30'
            when t.num>30 and t.num<=40 then 'a_30_40' else null end seq; 
  from table t
) group by seq

------解决方案--------------------
探讨

SQL code


select case when num >= 10 and num < 15 then '10~15'
            when num >= 15 and num < 30 then '15~30'
            when num >= 30 and num < 40 then '30~40'
            else null
 ……


------解决方案--------------------
引用: 
  
 SQL code 
  
  
 select case when num > = 10 and num  < 15 then  '10~15 ' 
 when num > = 15 and num  < 30 then  '15~30 ' 
 when num > = 30 and num  < 40 then  '30~40 ' 
 else null 
 …… 
  
 2楼的++