mysql - 如何在sql中的两个不同列中查找值的频率？

Question

我有一张表格，显示航班的到达机场和出发机场。我需要找到每个机场的到达和离开

第一个是给定的表，第二个是所需的表

我试过以下

SELECT T.departure, count(F.departure), count(F.arrival) 
FROM (
    SELECT departure FROM flights 
    UNION 
    SELECT arrival FROM flights
) T
LEFT JOIN flights F ON T.departure  = F.departure
LEFT JOIN flights F2 ON T.departure = F.arrival
GROUP BY T.departure

Answer 1

这是一个选项：

SQL> with
  2  test (dep, arr) as
  3    -- your sample data
  4    (select 'a', 'e' from dual union all
  5     select 'a', 'e' from dual union all
  6     select 'a', 'e' from dual union all
  7     select 'b', 'f' from dual union all
  8     select 'b', 'f' from dual union all
  9     select 'b', 'g' from dual union all
 10     select 'c', 'g' from dual
 11    ),
 12  deparr as
 13    -- departures and arrivals
 14    (select dep airport, 1 departure, 0 arrival from test
 15     union all
 16     select arr airport, 0 departure, 1 arrival from test
 17    )
 18  select airport, sum(departure) departure, sum(arrival) arrival
 19  from deparr
 20  group by airport
 21  order by airport;

A  DEPARTURE    ARRIVAL
- ---------- ----------
a          3          0
b          3          0
c          1          0
e          0          3
f          0          2
g          0          2

6 rows selected.

SQL>

（顺便说一句，您的预期输出似乎是错误的。B 有 3 次出发。）