这是一条sql 语句,表示如果有就不执行,如果没有就将值赋空

declare @var nvarchar(1000)
set @var='select p.评级,sum((p.止点桩号 - p.起点桩号)) as 长度,sum((p.止点桩号 - p.起点桩号))*100/sum((l.止点桩号 - l.起点桩号)) as 比例,p.检测方向
from pjjg_mqi p,lxjbxx l group by p.检测方向,p.评级'
--,p.评级
if not exists (
select 评级

from pjjg_mqi

where 评级='差' and 检测方向=1

)
begin
set @var=@var + '
union
select ''差'',0,0,1'
end

if not exists (
select 评级

from pjjg_mqi

where 评级='差' and 检测方向=2

)
begin
set @var=@var + '
union
select ''差'',0,0,2'
end

if not exists (
select 评级

from pjjg_mqi

where 评级='次' and 检测方向=1

)
begin
set @var=@var + '
union
select ''次'',0,0,1'
end

if not exists (
select 评级

from pjjg_mqi

where 评级='次' and 检测方向=2

)
begin
set @var=@var + '
union
select ''次'',0,0,2'
end

if not exists (
select 评级

from pjjg_mqi

where 评级='中' and 检测方向=1

)
begin
set @var=@var + '
union
select ''中'',0,0,1'
end
if not exists (
select 评级

from pjjg_mqi

where 评级='中' and 检测方向=2

)
begin
set @var=@var + '
union
select ''中'',0,0,2'
end

if not exists (
select 评级

from pjjg_mqi

where 评级='良' and 检测方向=1

)
begin
set @var=@var + '
union
select ''良'',0,0,1'
end

if not exists (
select 评级

from pjjg_mqi

where 评级='良' and 检测方向=2

)
begin
set @var=@var + '
union
select ''良'',0,0,2'
end
if not exists (
select 评级

from pjjg_mqi

where 评级='优' and 检测方向=1

)
begin
set @var=@var + '
union
select ''优'',0,0,1'
end

if not exists (
select 评级

from pjjg_mqi

where 评级='优' and 检测方向=2

)
begin
set @var=@var + '
union
select ''优'',0,0,2'
end
set @var=@var+' order by p.评级,p.检测方向'
exec (@var)