在SQL子查询选择

Question

我正在尝试回答SQL问题以进行修订，但似乎无法解决如何使其工作。有问题的表格是：

问题是要求我编写一个SQL命令来显示每个员工的总距离超过100，员工姓名和员工在所有旅程中使用的总升数（数量）旅程的升数是distanceInKm / kmPerLitre）。

到目前为止，我已尝试过以下几种代码变体：

SELECT 
    name, TravelCost.distanceInKm / Car.kmPerLitre AS "Cost in Litres"
FROM 
    Employee, Car, TravelCost
WHERE 
    Employee.id = TravelCost.employeeID
    AND Car.regNo = TravelCost.carRegNo

在这一点上，我有点卡住，任何帮助都会非常感激，谢谢！

Answer 1

从不在FROM子句中使用逗号。 始终使用正确的，标准的，明确的JOIN语法。

您错过了GROUP BY和HAVING：

SELECT e.name, SUM(tc.distanceInKm / c.kmPerLitre) AS "Cost in Litres"
FROM Employee e JOIN
     TravelCost tc 
     ON e.id = tc.employeeID JOIN
     Car c
     ON c.regNo = tc.carRegNo
GROUP BY e.name
HAVING SUM(tc.distanceInKm) > 100;

Answer 2

使用分组和有条款

SELECT NAME,
       Sum(TravelCost.distanceInKm/ Car.kmPerLitre) AS "Cost in Litres"
FROM   Employee
       INNER JOIN TravelCost
               ON Employee.id = TravelCost.employeeID
       INNER JOIN Car
               ON Car.regNo = TravelCost.carRegNo
GROUP  BY NAME
HAVING Sum(distanceInKm) > 100 

Answer 3

您需要加入所有表并找到这样的升数：

providers: [DataService]

此外，您需要按ID分组，而不是像其他答案所示的名称分组。可以有多名员工同名。

此外，使用显式JOIN语法而不是旧的基于逗号的语法。它现代而且更加清晰。

Answer 4

-- **How fool am I! How arrogant am I! I just thought `sum(tc.distanceInKm/c.kmPerLitre)` 
-- may have a problem, since a employee may have multiple cars,and car's kmPerLitre is differenct. 
-- However there is no problem, it's simple and right! 
-- The following is what I wrote, what a bloated statement it is! **

-- calcute the total number of litres used by the employee on all journeys
select  e.name, sum(Cost_in_Litres) as "Cost in Litres"
from (
    select t.employeeID
        -- calcute the litres used by the employee on all journeys group by carRegNo
        , sum(t.distanceInKm)/avg(c.kmPerLitre) as Cost_in_Litres
    from TravelCost t
    inner join Car c
        on c.regNo = t.carRegNo
    where t.employeeID in 
    ( -- find the employees who has a total distance from all journeys of more than 100
    select employeeID 
        from TravelCost
        group by employeeID 
        having sum(distanceInKm)> 100
    )
    group by t.carRegNo, t.employeeID
) a
inner join Employee e
    on e.id = a.employeeID
group by e.id,e.name;