Mysql Sum 不适用于单行结果

Question

我有这个查询

SELECT product.product_id, 
                    product.product_category_id, 
                    product.product_subcategory_id, 
                    product.product_brand_id, 
                    product.product_model_id, 
                    product.product_retail_price, 
                    product.product_wholesale_price, 
                    product.product_quoted_price, 
                    product.product_aquoted_price, 
                    product.product_dquoted_price, 
                    (SELECT IFNULL(SUM(product_sold.product_quantity), 0) FROM product_sold WHERE product_sold.product_id = product.product_id 
                    AND product_sold.product_sold_approved = 1
                    AND product_sold.product_sold_approved_time > $start_timestamp) AS num_product_sold,
                    product_brand.brand_name, 
                    product_model.model_name, 
                    product_subcategory.subcategory_name,
                    (SELECT IFNULL(SUM(product_stock.product_quantity),0) FROM product_stock WHERE product_stock.product_id = product.product_id) AS num_product_stock
    FROM product
        INNER JOIN product_brand 
            ON product_brand.brand_id = product.product_brand_id
        INNER JOIN product_model 
            ON product_model.model_id = product.product_model_id AND product_model.year_from <= $year AND product_model.year_to >= $year
        INNER JOIN product_subcategory 
            ON product_subcategory.subcategory_id = $subcategory
        GROUP BY product.product_id
        ORDER BY num_product_sold DESC, num_product_stock DESC, product_brand.brand_name, product_model.model_name

问题是，当产品库存在库存表上只有一行时，它返回 0，我会在之后执行此查询，但问题是我需要按库存数量订购。有什么方法可以对产品数量求和，即使数据库中只有一行？

Answer 1

我已经尝试过你提到的方法，但我没有发现任何问题:

    -- create tables
    create table product(
        id number not null,
        name varchar2(50) not null,
        quantity number default 0 not null);

    create table product_sold(
        id number not null,
        product_id number not null,
        quantity number default 0 not null);    

    -- insert some data    
    insert into product values(1, 'porsche', 55);
    insert into product values(2, 'bmw', 18);
    insert into product values(3, 'airbus A330', 2); 


    insert into product_sold values(100,1, 3);
    insert into product_sold values(101,1, 15);
    insert into product_sold values(102,2, 6);

    commit;

    -- start querying

    SELECT product.id,product.name,(SELECT COALESCE(SUM(ps.quantity), 0) FROM product_sold ps WHERE ps.product_id = product.id) sum_sold
    FROM  product;

ID               NAME                 SUM_SOLD                              
1                porsche              18                                    
2                bmw                  6                                     
3                airbus A330          0