SQL：基于多列计数

Question

我有一张包含以下结构的表格。

Flight_ID      |     Source_city    |    DestinatinCity
    1                  NYC                  LONDON
    2                 LONDON                TOKYO
    3                 LONDON                 NYC

我想找到城市数量，即涉及源和目的地的城市，

city      | count
LONDON        3
NYC           2
TOKYO         1

如何使用基本SQL功能解决它（没有PL）。

Answer 1

使用union all和聚合。

select city,count(*) as cnt
from (
select flight_Id,source_city as city from t
union all 
select flight_Id,destination_city from t
) x
group by city

Answer 2

您可以使用union all

来实现它

select source_city as city, count(*) from table_name union all select dest_city,count(*) from table_name group by source_city;