Question

我有两个SQL表：cars和lap_times。

cars: 
- car_id
- engine_size
- car_type

lap_times
- lap_time_id
- time
- car_id

我希望得到以下内容： - 每辆车的车型数量 - 每辆车的平均发动机尺寸 - 每辆车中所有车型的最快圈数

我有这个：

SELECT 
    COUNT(cars.car_id), 
    AVG(cars.engine_size), 
    lps.FASTEST_LAP 
FROM 
    cars, 
    (SELECT lap_times.car_id AS C_ID, MIN(lap_times.time) AS FASTEST_LAP FROM lap_times) as lps 
WHERE 
    cars.car_id = lps.C_ID 
GROUP BY 
    cars.car_type

但我没有得到任何东西。不过，我没有收到任何错误。有什么想法有什么不对吗？

Answer 1

问题在于您的子查询，您希望子查询返回最快的car_id并返回一些car_id。

我建议您了解GROUP BY and HAVING with Hidden Columns

此外，如果您的子查询按预期运行，则只会选择一辆车而不是car_type的一辆车。

这是作业吗？你应该标记它。

Answer 2

试试这个：

SELECT  a.Car_Type, 
        COUNT(a.car_ID) as NoOfCars,
        AVG(a.engine_size) as AverageEngine,
        MIN(b.time) as Fastest
FROM    cars a INNER JOIN lap_times b ON
            a.Car_ID = b.Car_ID
GROUP BY a.car_id

Answer 3

尝试以下方法：

SELECT cars.car_type,
       COUNT(cars.car_id),
       AVG(cars.engine_size),
       MIN(lps.FASTEST_LAP)
  FROM cars,
       lap_times
 WHERE cars.car_id = lps.C_ID
 GROUP BY cars.car_type

Answer 4

with CTE as(
    select car_type, count(*) as count_cars, avg(engine_size) as avg_engine
    from cars C
    group by car_type
)
select * , 
     (SELECT MIN(time) FROM lap_times where car_id in (select car_id from cars where car_type=CTE.car_type))  
from CTE

Answer 5

没关系。通过在子查询内部进行分组来修复它。

SELECT  
    COUNT(cars.car_id),  
    AVG(cars.engine_size),  
    lps.FASTEST_LAP  
FROM  
    cars,  
    (SELECT lap_times.car_id AS C_ID, MIN(lap_times.time) AS FASTEST_LAP FROM lap_times GROUP BY lap_times.car_id) as lps
WHERE  
    cars.car_id = lps.C_ID  
GROUP BY  
    cars.car_type

为什么带有子查询的SQL查询不起作用？

5 个答案: