这样的查询如何写
------------------------------
品名 单价 地域别
a 10 1
a 11 2
b 5 1
b 4 2
c 3 1
d 6 2

想要的结果：
如果给条件是True则显示：
------------------------------
品名 单价 地域别
a 10 1
b 5 1
c 3 1
d 6 2
如果给条件是False则显示：
------------------------------
品名 单价 地域别
a 11 2
b 4 2
c 3 1
d 6 2

就是说：①如果条件为真，同一种品名有两种单价就显示地域别为1的单价，
一种品名只有一种单价就不考虑地域别
②如果条件为假，同一种品名有两种单价就显示地域别为2的单价，
一种品名只有一种单价就不考虑地域别
[解决办法]

--true
;with a1 (品名,单价,地域别) as
(
select 'a',10,1 union all 
select 'a',11,2 union all 
select 'b',5,1 union all 
select 'b',4,2 union all 
select 'c',3,1 union all 
select 'd',6,2
)
,a2 as
(
select *,row_number() over(partition by 品名 order by 地域别) re from a1
)
select 品名,单价,地域别
from a2 
where re=1

--False
;with a1 (品名,单价,地域别) as
(
select 'a',10,1 union all 
select 'a',11,2 union all 
select 'b',5,1 union all 
select 'b',4,2 union all 
select 'c',3,1 union all 
select 'd',6,2
)
,a2 as
(
select *,row_number() over(partition by 品名 order by 地域别 desc) re from a1
)
select 品名,单价,地域别
from a2 
where re=1

create table kc
(品名 varchar(5),单价 int,地域别 int)

insert into kc
 select 'a',10,1 union all
 select 'a',11,2 union all
 select 'b',5,1 union all
 select 'b',4,2 union all
 select 'c',3,1 union all
 select 'd',6,2


-- 如果给条件是True
declare @x varchar(10)
select @x='True'

select a.* 
 from kc a
 where a.地域别=case @x when 'True' then 1
                       when 'False' then 2 end
 or not exists
 (select 1 from kc b where b.品名=a.品名 and b.地域别!=a.地域别)

/*
品名    单价         地域别
----- ----------- -----------
 a     10          1
 b     5           1
 c     3           1
 d     6           2

(4 row(s) affected)
*/


-- 如果给条件是False
declare @x varchar(10)
select @x='False'

select a.* 
 from kc a
 where a.地域别=case @x when 'True' then 1
                       when 'False' then 2 end
 or not exists
 (select 1 from kc b where b.品名=a.品名 and b.地域别!=a.地域别)

/*
品名    单价         地域别 
 
----- ----------- -----------
 a     11          2
 b     4           2
 c     3           1
 d     6           2

(4 row(s) affected)
*/

create table kc
(品名 varchar(5),单价 int,地域别 int)
 
insert into kc
 select 'a',10,1 union all
 select 'a',11,2 union all
 select 'b',5,1 union all
 select 'b',4,2 union all
 select 'c',3,1 union all
 select 'd',6,2
 go
 

declare @condition varchar(10)

set @condition='true'  --按照你的需要,输入true或者false

select 品名,单价,地域别
from
(
select *,
       ROW_NUMBER() over(partition by 品名 
                             order by case when @condition = 'true' and 地域别 = 1
                                                then 0
                                           when @condition = 'false' and 地域别 = 2
                                                then 0
                                           else 1
                                      end) as rownum
from kc
)t
where rownum = 1
/*
品名单价地域别
a101
b51
c31
d62
*/

IF OBJECT_ID(N'kc',N'U') IS NOT NULL
    DROP TABLE kc
  
 CREATE TABLE kc
    (
      品名 VARCHAR(5) ,
      单价 INT ,
      地域别 INT
    )
insert into kc
 select 'a',10,1 union all
 select 'a',11,2 union all
 select 'b',5,1 union all
 select 'b',4,2 union all
 select 'c',3,1 union all
 select 'd',6,2
 
-- 如果给条件是True
DECLARE @C varchar(10)
DECLARE @Sql NVARCHAR(MAX)
SET @C='True'

-- 如果给条件是False
--SET @C='False'

SET @Sql='SELECT c.品名,c.单价,c.地域别 FROM
( 
 SELECT CASE '''+@C+''' WHEN ''True''  THEN MIN(ID) ELSE MAX(ID) END ID,品名 FROM
 (
 SELECT ROW_NUMBER() OVER(ORDER BY 品名) AS ID ,* FROM kc)a GROUP BY 品名)b,(SELECT ROW_NUMBER() OVER(ORDER BY 品名) AS ID ,* FROM kc)c
 WHERE b.ID=c.ID'
EXEC(@sql)
  
  品名    单价          地域别
----- ----------- -----------
a     10          1
b     5           1
c     3           1
d     6           2

(4 行受影响)