case when 覓語 れ螻 螳 企.
case 
	when <= 10 then '1 ~ 10'
	when <= 20 then '11 ~ 20'
	else '21 ~
end 

蠏碁 企蟆 . 狩襾殊 覓語螳 蟇一 .
襷 願 蠑語螻, 譟郁企朱 焔レ る慨螻 る 一危一 覿襯 螻ろ伎 .
襯 れ,

drop table #temp
create table #temp(num int)
go

insert #temp 
select top 10000000 1
from master..spt_values a
    cross join master..spt_values b
    cross join master..spt_values c
    cross join master..spt_values d
    cross join master..spt_values e
go
insert #temp 
select top 5000000 2
from master..spt_values a
    cross join master..spt_values b
    cross join master..spt_values c
    cross join master..spt_values d
    cross join master..spt_values e
go
insert #temp 
select top 1000000 3
from master..spt_values a
    cross join master..spt_values b
    cross join master..spt_values c
    cross join master..spt_values d
    cross join master..spt_values e
go

select
    num
,   count(*) cnt
from #temp
group by
    num
order by 1

select 蟆郁骸
num         cnt
----------- -----------
1           10000000
2           5000000
3           1000000

覿襯 螻ろ伎 譟郁唄 覦一 覃 所 焔 レ 蠍磯 .
譟郁唄 覲蟆渚 100 ろ.
--drop table #rs
create table #rs(
    grp char(1)
,   cnt int
)

--drop table #stat
create table #stat(method int, exec_time int)


set nocount on
set statistics io off
set statistics time off

--覦覯1
declare 
    @i int = 1
,   @begin_dt datetime
,   @end_dt datetime

truncate table #rs;
while(@i <= 100)
begin 
    set @begin_dt = getdate()

    insert #rs
    select
        case 
            when num = 1 then 'a'
            when num = 2 then 'b'
            else 'c'
        end
    ,   count(*) cnt
    from #temp
    group by
        case 
            when num = 1 then 'a'
            when num = 2 then 'b'
            else 'c'
        end   
    set @end_dt = getdate()

    insert #stat
    select 1, datediff(ms, @begin_dt, @end_dt)

    set @i += 1
end
go    


--覦覯2
declare 
    @i int = 1
,   @begin_dt datetime
,   @end_dt datetime

truncate table #rs;
while(@i <= 100)
begin 
    set @begin_dt = getdate()

    insert #rs
    select
        case 
            when num = 3 then 'c'
            when num = 2 then 'b'
            else 'a'
        end
    ,   count(*) cnt
    from #temp
    group by
        case 
            when num = 3 then 'c'
            when num = 2 then 'b'
            else 'a'
        end   
    
    set @end_dt = getdate()

    insert #stat
    select 2, datediff(ms, @begin_dt, @end_dt)

    set @i += 1
end
go

豸′ 企慨覃..
select
    method 覦覯
,   avg(exec_time) 蠏
,   stdev(exec_time) 譴ク谿
from #stat
group by
    method 

覦覯2螳 譟郁 ろ螳 蠍 蟆 覲 .
覦覯          蠏          譴ク谿
----------- ----------- ----------------------
1           507         143.08132144153
2           521         123.3129123143


num=1 企豪 1豌襷 蟇伎願, num=3 1覦焔 蟇伎企. 蠏碁覩襦 螳 覃 豌 覯讌 when num = 1 then 'a' 覿覿 蟇碁Μ蟆 .
        case 
            when num = 1 then 'a' --------(1)
            when num = 2 then 'b'
            else 'c'              --------(3)
        end 

讌襷, 螳危覃 num=1 企豪 1豌襷 蟇伎 譟郁唄 螳螳 else蟾讌 企り . 讀, if襯 覈 覯 磯 谿企.
        case 
            when num = 3 then 'c'
            when num = 2 then 'b'
            else 'a'
        end