如何计算查询内列的平均值并显示它

Question

如何选择所有产品、产品名称、平均星级和最近的评分日期？结果应按平均星级的降序排列。 这就是我到目前为止所做的

SELECT product_name, AVG(rating_stars)
FROM rating
INNER JOIN product
ORDER BY AVG(rating_stars) DESC

这就是数据库

CREATE TABLE product
(
    prod_id varchar(15),
    product_name varchar(30),
    product_price double precision,
    product_manufacturer varchar(30),
    PRIMARY KEY (prod_id)
)

CREATE TABLE customer
(
    cust_id varchar(15),
    customer_name varchar (30),
    customer_adress varchar (30),
    PRIMARY KEY (cust_id)
)

CREATE TABLE rating
(
    prod_id,
    cust_id,
    rating_date date not null,
    rating_stars int (5),

    PRIMARY KEY (rating_date),
    FOREIGN KEY (prod_id) REFERENCES Products(prod_id),
    FOREIGN KEY (cust_id) REFERENCES customer(cust_id)
)

CREATE TABLE sale
(
    sale_id,
    prod_id,
    cust_id,
    sale_quantity int,
    sale_cost double precision,

    PRIMARY KEY (sale_id),
    FOREIGN KEY (prod_id) REFERENCES Products(prod_id),
    FOREIGN KEY (cust_id) REFERENCES customer(cust_id)
)

Answer 1

你可以使用 group by 和 max

SELECT product_name, AVG(rating_stars),  max(rating_date)
FROM rating
INNER JOIN product on rating.prod_id = product.prod_id 
GROUP BY product_name
ORDER BY AVG(rating_stars) DESC