如何根據另一個表的PK獲取表的兩列計數？

Question

我有兩個表定義為：

CREATE TABLE airports (
  code char(3) not null primary key,
  name varchar(100) not null,
  city varchar(50) not null,
  state varchar(5),
  country char(2) not null references fcountries(code)
)

和

CREATE TABLE flights (
  departure char(3) not null references airports(code), -- airport code
  arrival char(3) not null references airports(code), -- airport code
  dep_time time not null,
  airline char(2) not null references airlines(code)
)

我需要使用 postgres 獲取從機場起飛和抵達機場的航班數量。 所以，我寫了

select
  code,
  count(departure)
from airports 
join flights 
 on flights.departure = airports.code group by code;

出發時同樣如此，

select
  code,
  count(arrival) as Arrival
from airports 
join flights 
  on flights.arrival = airports.code group by code;

但是我需要將這兩者結合起來，並在相同的結果中獲得到達-離開計數。 我不知道該怎么做？ 我寫了這個查詢

select
  code,
  count(departure),
  x.arrival
from (
  select count(arrival) as arrival
  from airports 
  join flights 
    on flights.arrival = airports.code group by code
) x, 
airports 
join flights on airports.code = flights.departure
group by code, x.arrival
order by code;

但結果不正確，因為在此結果中重復了“代碼”。 我對 sql 很陌生，不知道如何寫這個。

Answer 1

最簡單的方法是橫向連接和group by ：

select v.code, count(*) as total,
       sum(is_departure) as num_departures,
       sum(is_arrival) as num_arrivals
from flights cross join lateral
     (values (departure, 1, 0), (arrival, 0, 1)
     ) as v(code, is_departure, is_rrival)
     on f.departure = a.code
group by v.code;

請注意， JOIN to airports是不必要的，除非您確實想引入問題中未提及的更多信息。

Answer 2

您可以將這兩個查詢用作子查詢並加入

    select t1.code,  t1.count_departure, t2.count_arrival 
    from (
        select code, count(departure)  count_departure
        from airports join flights on flights.departure = airports.code 
        group by code

    ) t1  
    inner join  (
        select code, count(arrival) as  count_arrival 
        from airports join flights on flights.arrival = airports.code 
        roup by code

    ) t2 on t1.code  = t2.code ;

Answer 3

我認為在加入后不能像這樣使用別名 t2 這就是為什么你會收到錯誤。 嘗試這個：

select t1.code, t1.count_departure, t2.count_arrival from ( select code, count(departure) count_departure from airports join flights on flights.departure = airports.code group by code ) t1 join ( select code, count(arrival) as count_arrival from airports join flights on flights.arrival = airports.code group by code ) t2 on t1.code = t2.code;

Answer 4

一個非常簡單的方法是在 SELECT 子句中使用相關的 COUNT 子查詢：

select a.code,
    (select count(*) from flights f where f.departure = a.code) as departures,
    (select count(*) from flights f where f.arrival = a.code) as arrivals
from airports a;