计算来自多个列的不同值

Question

给定以下架构：

id departure arrival
0  BOS       LAX
1  SFO       SEA
2  MIA       LAX
3  RDU       BOS
4  JFK       DEN
5  LAX       SEA

我需要计算每个机场的总数。 例如，BOS应该为2（一次起飞一次到达）。

我可以通过两个单独的查询来做到这一点：

SELECT departure, COUNT(*) FROM legs
GROUP BY departure ORDER BY COUNT(departure) DESC

和

SELECT arrival, COUNT(*) FROM legs
GROUP BY arrival ORDER BY COUNT(arrival) DESC

但我无法在一个查询中找到解决办法。 我想要以下内容：

airport count
BOS     2
LAX     2
SEA     2
JFK     1

Answer 1

用union ：

select departure as airport, count(*) as count
from (select departure from legs
      union all
      select arrival from legs)t
group by departure

Answer 2

在两个单独的聚合上使用FULL [OUTER] JOIN ：

SELECT airport, COALESCE(d.ct, 0) + COALESCE(a.ct, 0) AS "count"
FROM  (
   SELECT departure AS airport, count(*) AS ct
   FROM   legs
   GROUP  BY 1
   ) d
FULL JOIN (
   SELECT arrival AS airport, count(*) AS ct
   FROM   legs
   GROUP  BY 1
   ) a USING (airport)
ORDER  BY "count" DESC, airport;

这样，您可以轻松地返回到达和离开的其他列，并且如果您想选择某些机场，则可以在基表上使用索引。

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