計數用兩個表，並顯示爲0沒有結果SQL

Question

我一直在掙扎，真的很感激一些幫助：

我有兩個表航班和載體。

航班

Flight_ID | Airport_Origin | Airport_Destination | Carrier_ID 
1 | DOM | IAD | 1 
2 | IAD | DOM | 4 
3 | BAL | UEI | 1 
4 | LAX | CHI | 2 
5 | VAS | SDA | 3 
6 | MUM | PAR | 2 
7 | LAD | BOS | 4 

運營商

carrier_ID | Carrier_Name 
1 | American 
2 | southwest 
3 | delta 
4 | united 
5 | spirit 

我想和航班數量的計數來顯示它的名字各承運航空公司。如果航空公司沒有航班，它應該顯示0，並應顯示所有運營商。

輸出應該是：

CarrierName | NumberofFlights 
American | 2 
southwest | 2 
delta  | 1 
united  | 2 
spirit  | 0 

我的代碼：

select carriers.Carrier_Name AS 'Carrier Name', count(*) AS 'Number of Flights' 
from flights 
inner join carriers 
on flights.Carrier_ID = carriers.Carrier_ID 
group by Carrier_Name 
Order BY Carrier_Name 
; 

我似乎無法顯示，沒有航班爲0.我的輸出爲載體如下：

Answer 1

這是一個LEFT JOIN（外連接）當記錄從加入可能不存在的右側部分：

select 
    c.Carrier_Name AS 'Carrier Name', count(f.Flight_ID) AS 'Number of Flights' 
from carriers c 
left join flights f on f.Carrier_ID = c.Carrier_ID 
group by Carrier_Name 
order by Carrier_Name 

也這可通過這樣的子查詢來完成：

select 
    c.Carrier_Name AS 'Carrier Name', 
    (select count(*) from flights f where f.Carrier_ID = c.Carrier_ID) AS 'Number of Flights' 
from carriers c 
order by Carrier_Name 

INNER JOIN意味着來自兩個部分對應的記錄加入必須存在。

Answer 2

您需要使用外連接，以便結果包含不匹配的行。然後COUNT函數必須對flights表中的一行進行計數，以便它不會計算不匹配時產生的空行。

SELECT c.carrier_name AS `Carrier Name`, COUNT(f.Flight_ID) AS `Number of Flights` 
FROM carriers AS c 
LEFT JOIN flights AS f ON f.carrier_ID = c.carrier_ID 
GROUP BY c.carrier_name 
ORDER BY c.carrier_name