如何能在同一個表合併兩列，計算其獨特的出現

Question

我一直在掙扎，真的很感激一些幫助：

我有兩個表車和騎

cars 
car_id car_manuf car_model 
    1 Honda  CRV 
    2 Honda  Accord 
    3 Toyota  Corolla 
    4 Toyota  Camry 
    5 Ford  Fusion 

rides 
ride_id car_id ride_destination 
     1  3 Boston 
     2  5 New York 
     3  5 Washington DC 
     4  1 California 
     5  2 Dallas 
     6  5 Canada 

我想請計算每個車型的乘車次數，該車型將包含car_manuf和car_model的組合，並且應該按照從最多到最少的次數排序。

輸出應該是：

CarType-NumberofRides 
Honda_CRV-1 
Honda_Accord-1 
Toyota_Corolla-1 
Toyota_Camry-0 
Ford_Fusion-3 

用最少量的遊樂設施

排序輸出

CarType-NumberofRides 
Toyota_Camry-0 
Honda_Accord-1 
Toyota_Corolla-1 
Honda_CRV-1 
Ford_Fusion-3 

mycode的：

select 
    c.car_manuf + '_' + c.car_model AS 'Car Type', 
    (select count(*) from rides r where r.car_id = c.car_id) AS 'Number of Rides' 
from cars c; 

我還挺這裏，而不是陷入不確定哪個方向我應該去關於獲得正確的輸出。

Answer 1

當出現問題時，您必須使用GROUP BY和ORDER BY。我使用CONCAT連接字符串而不是+號。使得正在發生的事情變得更清楚，並且不會被誤認爲算術運算。

SELECT CONCAT(c.car_manuf, '_', c.car_model) AS CarType, COUNT(r.car_id) AS NumberOfRides 
FROM rides r 
LEFT JOIN cars c ON (r.car_id = c.car_id) 
GROUP BY CarType 
ORDER BY NumberOfRides ASC 

然而，這忽略了0次出現。

如果你想看到的0以及交換表以：

SELECT CONCAT(c.car_manuf, '_', c.car_model) AS CarType, COUNT(r.car_id) AS NumberOfRides 
FROM cars c 
LEFT JOIN rides r ON (r.car_id = c.car_id) 
GROUP BY CarType 
ORDER BY NumberOfRides ASC