Question

我一直在努力，非常感谢一些帮助：

我有两张桌子车和游乐设施

ruby http://www.example.com/script.rb

汽车输出：create table cars (cars_ID Int (10), manuf varchar (30), model varchar (30)); create table rides (ride_ID Int (10), cars_ID varchar (30), ride_dest varchar (30)); insert into cars (cars_ID, manuf, model) values (1, 'Honda', 'CRV'); insert into cars (cars_ID, manuf, model) values (2, 'Ford', 'Fusion'); insert into cars (cars_ID, manuf, model) values (3, 'Toyota', 'Corolla'); insert into cars (cars_ID, manuf, model) values (4, 'Toyota', 'Camry'); insert into cars (cars_ID, manuf, model) values (5, 'Ford', 'Fusion'); insert into cars (cars_ID, manuf, model) values (6, 'Ford', 'Fusion'); insert into cars (cars_ID, manuf, model) values (7, 'Ford', 'Fusion'); insert into cars (cars_ID, manuf, model) values (8, 'Ford', 'Fusion'); insert into cars (cars_ID, manuf, model) values (9, 'Ford', 'Fusion'); insert into rides (ride_ID, cars_ID, ride_dest) values (1, 3, 'Boston'); insert into rides (ride_ID, cars_ID, ride_dest) values (2, 5, 'New York'); insert into rides (ride_ID, cars_ID, ride_dest) values (3, 5, 'Washington DC'); insert into rides (ride_ID, cars_ID, ride_dest) values (4, 1, 'California'); insert into rides (ride_ID, cars_ID, ride_dest) values (5, 2, 'Dallas'); insert into rides (ride_ID, cars_ID, ride_dest) values (6, 5, 'Canada'); insert into rides (ride_ID, cars_ID, ride_dest) values (7, 5, 'Canada'); insert into rides (ride_ID, cars_ID, ride_dest) values (8, 1, 'Canada'); insert into rides (ride_ID, cars_ID, ride_dest) values (9, 5, 'Canada'); insert into rides (ride_ID, cars_ID, ride_dest) values (10, 3, 'Canada');

select * from cars;

游乐设施的输出：cars_ID-manuf-model 1-Honda-CRV 2-Ford-Fusion 3-Toyota-Corolla 4-Toyota-Camry 5-Ford-Fusion 6-Ford-Fusion 7-Ford-Fusion 8-Ford-Fusion 9-Ford-Fusion

select * from rides;

我想计算每种车型的乘坐次数（car_manuf和car_model的组合）输出应该显示每种车型的乘坐总和，并且应该从最多到最少的乘坐次数进行排序。

输出应为：

ride_ID-cars_ID-ride_dest

1-3-Boston
2-5-New York
3-5-Washington
4-1-California
5-2-Dallas
6-5-Canada
7-6-Canada
8-7-Canada
9-8-Canada
10-9-Canada

mycode：没有GROUP BY

CarType-NumberofRides

Ford-Fusion - 8
Toyota-Corolla - 1
Honda-CRV - 1
Toyota-Camry - 0

myOutput：没有GROUP BY

SELECT CONCAT(c.manuf, '-', c.model) AS 'CarType',
    (select count(*) from rides r where r.cars_ID = c.cars_ID) AS 'NumberOfRides'
from cars c
order by NumberofRides ASC;

mycode：with GROUP BY

CarType-NumberOfRides
Toyota-Camry - 0
Toyota-Corolla - 1 
Ford-Fusion - 1
Honda-CRV - 1

myOutput：没有GROUP BY

SELECT CONCAT(c.manuf, '-', c.model) AS 'CarType',
    (select count(*) from rides r where r.cars_ID = c.cars_ID) AS 'NumberOfRides'
from cars c
GROUP BY CarType
order by NumberofRides ASC;

这不是我要找的输出。相反，所有CarType都应显示游乐设施的总数。

正确的输出是：

CarType-NumberOfRides

Toyota-Camry - 0
Toyota-Corolla - 1 
Ford-Fusion - 1
Ford-Fusion - 1
Ford-Fusion - 1
Honda-CRV - 1
Ford-Fusion - 1
Ford-Fusion - 1
Ford-Fusion - 3

如何获得正确的输出？我是否必须使用SUM，如果是这样，它将如何放置？

Answer 1

您不需要子查询，您应该加入这两个表。

SELECT CONCAT(c.manuf, '-', c.model) AS CarType, COUNT(r.ride_ID) AS NumberofRides
FROM cars AS c
LEFT JOIN rides AS r ON r.cars_ID = c.cars_ID
GROUP BY CarType
ORDER BY NumberofRides DESC

LEFT JOIN确保没有游乐设施的汽车将包含在联接中。这些行中的相应ride_ID将为NULL，而COUNT(r.ride_ID)不会计算空值。

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