Question

我花了很多时间来构建这个选择，但我无法解决它。我有2张桌子。第一个表叫做car并且有PK（主键）id_car和另一个列名等等。第二个表称为评级，并具有colums id_rating（PK），id_car（FK）和rating_value（整数）。如你所料，一辆车可以有多个咆哮。我想选择所有车型，我想知道每辆车的平均评级。最后，我想通过这个平均desc来排序结果。我正在尝试这样的事情：

    SELECT id_car, name, average
    FROM car C, rating R 
    WHERE C.id_car = R.id_car 
    ORDER BY (average) (
        SELECT AVG(rating_value) AS average 
        FROM rating R
        WHERE C.id_car = R.id_car)

但它不起作用。

Answer 1

SQL Server;另外，我建议您使用JOIN代替WHERE table1, table2..

SELECT C.id_car, name, AVG(rating_value) AS average
FROM car C JOIN rating R 
      ON C.id_car = R.id_car
GROUP By C.id_car, name
ORDER BY average DESC

Answer 2

这实现了聚合函数AVG()，然后GROUP BY实现了汽车ID和名称：

select c.id_car, c.name, avg(r.rating_value) aver
from car c
left join rating r
  on c.id_car = r.id_car
group by c.id_car, c.name
order by aver desc

使用LEFT JOIN将包括结果中的所有汽车，即使那些尚未评级的汽车。

Answer 3

SELECT C.name, AVG(R.rating_value)
FROM car C, rating R 
WHERE C.id_car = R.id_car
GROUP BY C.name
ORDER BY AVG(R.rating_value) DESC

Answer 4

 SELECT c.id_car, c.name,avg(r.rating_value) as rating   
FROM car c
 join  rating r 
 on c.id_car = r.id_car   
group by r.id_car 
order by rating

SQL从另一个表中选择平均值

4 个答案: