如何根据另一个表的 PK 计算一个表的两列？答案

【问题标题】：How to get count of two columns of a table based on PK of another table?如何根据另一个表的 PK 计算一个表的两列？
【发布时间】：2019-07-11 03:56:36
【问题描述】：

我有两个表定义为：

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 获取从机场出发和到达机场的航班数量。所以，我写了

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

对于出发也是如此，

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

但我需要将这两者结合起来，并在相同的结果中获得到达-离开计数。我不知道该怎么做？我写了这个查询

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;

但结果不正确，因为在此结果中重复了“代码”。我对 sql 很陌生，不知道怎么写。

【问题讨论】：

更新您的问题，添加相关表架构一个适当的数据样本和预期结果
请决定 DBMS。
我猜错了。 group by code 而不是 roup by code
我改了还是有错误。

标签： mysql sql

【解决方案1】：

您可以将这两个查询用作子查询并加入

    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 ;

【讨论】：

这给出了错误“选择附近的语法错误”，即在 t2 之后选择（选择代码....）我不知道为什么？
答案已更新.. 别名的位置错误

【解决方案2】：

我认为别名 t2 不能在加入后那样使用，这就是您收到错误的原因。试试这个：

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;

【讨论】：

【解决方案3】：

最简单的方法是横向连接和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;

请注意，JOIN 到 airports 是不必要的，除非您确实想要引入更多您在问题中未提及的信息。

【讨论】：

【解决方案4】：

一个非常简单的方法就是在 SELECT 子句中使用相关的 COUNT 子查询：

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;

【讨论】：