当两个记录很常见时单行获取

Question

Fromjourney ToJourney   KM
Delhi       Mumbai      1000
Mumbai      Delhi       1000
Gurgaon     Noida       50
Noida       Gurgaon     50
Ghaziabad   Faridabad   60
Faridabad   Ghaziabad   60
Delhi       Meerut      80
Meerut      Delhi       80

我有三列from，To和KM。

当From To拥有相同的城市时，我想要单行数据 像From-Mumbai，To-Delhi      To-Delhi，From-Mumbai

Ι想要这一行的单行

Answer 1

select T1.Fromjourney,T1.ToJourney
  ,T2.*
from TestTable T1
left join TestTable T2 
  on T1.Fromjourney = T2.ToJourney
  and T1.ToJourney = T2.Fromjourney
  and T1.Fromjourney < T2.Fromjourney 
where T2.Fromjourney is not null

| Fromjourney | ToJourney | Fromjourney | ToJourney |   KM |
|-------------|-----------|-------------|-----------|------|
|       Delhi |    Mumbai |      Mumbai |     Delhi | 1000 |
|     Gurgaon |     Noida |       Noida |   Gurgaon |   50 |
|   Faridabad | Ghaziabad |   Ghaziabad | Faridabad |   60 |
|       Delhi |    Meerut |      Meerut |     Delhi |   80 |

DEMO - SQL Fiddle

Answer 2

我会这样做：

select t.*
from testtable t
where Fromjourney < ToJourney
union all
select t.*
from testtable t
where Fromjourney > ToJourney and
      not exists (select 1
                  from testtable t2
                  where t2.Fromjourney = t.ToJourney and t2.ToJourney = t.Fromjourney);