使用外连接难点

Question

我有问题使用外连接写这个问题的查询。我可以在没有外连接的情况下写这个，但是它给外连接带来一些困难。

问：

有关“2018年1月1日”每次飞行，发现所有这一切都没有保留的座位（使用外连接）。对于每个无保留的座位，请显示航班号和座位号。

我有如下表：

create table passenger ( 
    passenger_ID varchar (10) , 
    passenger_name varchar (30) , 
    passenger_city varchar (30) , 
    primary key (passenger_ID)) ; 

create table flight ( 
    flight_number varchar (10) , 
    departure_airport varchar (10), 
    arrival_airport varchar (10) , 
    primary key (flight_number)); 

create table seat ( 
flight_number varchar (10), 
seat_number varchar (10), 
primary key(flight_number, seat_number), 
foreign key(flight_number) references flight); 

create table reservation(
    passenger_ID varchar (10), 
         flight_number varchar (10), 
         seat_number varchar (10), 
         day date , 
         fare numeric(8,2), 
primary key (flight_number, seat_number,day), 
foreign key (flight_number, seat_number) references seat , 
foreign key (passenger_ID) references passenger); 

请看看这个SQL小提琴。我已经构建了这个模式。 [http://sqlfiddle.com/#!17/feaca/1

查询，我现在是：

Select distinct a.flight_number , a.seat_number 
from 
seat a 
left outer join 
reservation b 
on 
a.flight_number = b.flight_number 
and 
a.seat_number <> b.seat_number 
where b.day = to_date('2017-10-01','YYYY-MM-DD') 
order by 1; 

任何建议和帮助，将不胜感激。

Answer 1

您想要加入flight_number，seat_number以及给定日期。然后，您可以过滤预留中没有相应行的行。

Select distinct s.flight_number , s.seat_number 
from seat s 
left outer join reservation r 
    on r.flight_number = s.flight_number 
    and r.seat_number = s.seat_number 
    and r.day = to_date('2017-10-01','YYYY-MM-DD') 
join (
    select distinct flight_number 
    from reservation 
    where day = to_date('2017-10-01','YYYY-MM-DD') 
) r2 on s.flight_number = r2.flight_number 
where r.day is null 
order by 1; 

或使用IN：

Select distinct s.flight_number , s.seat_number 
from seat s 
left outer join reservation r 
    on r.flight_number = s.flight_number 
    and r.seat_number = s.seat_number 
    and r.day = to_date('2017-10-01','YYYY-MM-DD') 
where r.day is null 
and s.flight_number in (
    select flight_number 
    from reservation 
    where day = to_date('2017-10-01','YYYY-MM-DD') 
    ) 
order by 1;