avg和count可以在sql中一起工作吗？

Question

我的数据库中有两个表，它们看起来像这样

create table restaurants(
restID number(8) NOT NULL,(PK)
name varchar(50),
photo varchar(50))

和

create table review_Restaurants(
commentID number(8) NOT NULL,
reviewDate timestamp,
commentValue varchar(100),
rating number(1),
userName varchar (25) not null,
restID number(8) not null,);

我想做的是从数据库中获取最好的餐厅（更高的评分和评论）图像并将其显示在屏幕上。

我解决这个问题的方法是，我将计算每一个大于 3 的休息的平均评分，以及那些 cmets 超过 5 的人。

select ((avg)rating > 3) as "rating",(count(reviews) > 5) as "review"
from review_restaurant
group by restID;

但它不起作用。我做错了什么？

Answer 1

SELECT restID
     , Avg(rating) As "rating"
     , Count(reviews) As "review"
FROM   review_restaurant
GROUP
    BY restID
HAVING Avg(rating) > 3
AND    Count(reviews) > 5

如果您从上述语句中删除 HAVING 子句，它将返回所有餐厅评论及其平均评分和评论数量。

HAVING 子句与WHERE 子句非常相似，但细微的区别在于HAVING 子句在聚合发生后应用。

另一种方法是使用子查询

SELECT restID
     , rating
     , review
FROM   (
        SELECT restID
             , Avg(rating) As "rating"
             , Count(reviews) As "review"
        FROM   review_restaurant
        GROUP
            BY restID
       ) As a_subquery
WHERE  rating > 3
AND    reviews > 5

这里也适用与使用 HAVING 子句相同的逻辑...我们计算所有平均评分和评论数量，然后我们应用我们的标准。

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如果您想从restaurants 表中检索相应的信息，那么一旦我们需要在子查询之上进行查询并将其连接回restaurants 表。

SELECT restaurants.restID
     , restaurants.name
     , restaurants.photo
     , top_restaurants.rating As average_rating
     , top_restaurants.review As number_of_reviews
FROM   restaurants
 INNER
  JOIN (
        SELECT restID
             , Avg(rating) As "rating"
             , Count(reviews) As "review"
        FROM   review_restaurant
        GROUP
            BY restID
        HAVING Avg(rating) > 3
        AND    Count(reviews) > 5
       ) As top_restaurants
    ON top_restaurants.restID = restaurants.restID