計算來自多個列的不同值

Question

給定以下架構：

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

我需要計算每個機場的總數。 例如，BOS應該為2（一次起飛一次到達）。

我可以通過兩個單獨的查詢來做到這一點：

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

但我無法在一個查詢中找到解決辦法。 我想要以下內容：

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

在兩個單獨的聚合上使用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;

這樣，您可以輕松地返回到達和離開的其他列，並且如果您想選擇某些機場，則可以在基表上使用索引。

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