MySql中的分組和最大數目

Question

下午好！ 我開始學習MySql並遇到問題：

查詢錯誤：錯誤：ER_WRONG_FIELD_WITH_GROUP：SELECT列表的表達式＃1不在GROUP BY子句中，並且包含未聚合的列'test.product.name'，該列在功能上不依賴於GROUP BY子句中的列； 這與sql_mode = only_full_group_by不兼容

我使用這個網站來研究https://www.db-fiddle.com/f/xxV16kmnZKmPktUdMSK6xZ/0

我的任務是： 找到每種產品價格最低的地區。 當多個地區的價格相同時，請按字母順序選擇第一個。

我的桌子：

create table product(id int, name varchar(99));
create table region(id int, name varchar(99));
create table price(productId int, regionId int, price decimal(9, 2));

insert into product values(1, 'Crab');
insert into product values(2, 'Crayfish');

insert into region values(1, 'Kiev');
insert into region values(2, 'Kharkov');
insert into region values(3, 'Lvov');

insert into price values(1, 1, 100);
insert into price values(1, 2, 100);
insert into price values(1, 3, 200);
insert into price values(2, 1, 200);
insert into price values(2, 2, 100);
insert into price values(2, 3, 100); 

我的選擇：

select product.name 'product', price.price, region.name 'Region name' 
from price
left join product on product.id = price.productId
left join region on region.id = price.regionId
where price.price != 0
group by region.name
having max(price.price)
order by region.name

預期結果：

product region  price
Crab    Kharkov 100
Crayfish    Kharkov 100

Answer 1

要獲得每種產品的最低價格，您可以執行以下操作：

select p.*
from price p
where p.price = (select min(p2.price) from price p2 where p2.product_id = p.product_id);

其他join將引入名稱而不是id 。

要獲得每種產品一行：

select p.product_id, p.price, min(r.name)
from price p join
     region r
     on p.regionId = r.id
where p.price = (select min(p2.price) from price p2 where p2.product_id = p.product_id)
group by p.product_id, p.price;

Answer 2

您可以對按產品和地區分組的最高價格使用聯接

select t.product, price.price, t.region
from price
INNER JOIN ( 
select price.productID, price.regionId, product.name product, 
          max(price.price) max_price, region.name Region 
from price
left join product on product.id = price.productId
left join region on region.id = price.regionId
where price.price != 0
group by product.name , 
    region.name,
    price.productID,
    price.regionId ) t on t.productId = price.productId  
      and t.regionId = price.regionId and t.max_price =  price.price

Answer 3

我會用類似的東西：

select
  p2.name as product,
  (
    select min(r.name)
    from
      price as c2
      inner join region as r on c2.regionid = r.id
    where c2.productId = t.id and c2.price = t.price) as region,
  t.price
from
  product as p2
  inner join (
    select
      p.id,
      min(c.price) as price
    from
      product as p
      left outer join price as c on p.id = c.productId
    group by p.id) as t on p2.id = t.id

主要思想是獲得每種產品的最低價格（子查詢t），然后為每種產品/價格組合找到最小（即第一個字母）區域