按字段对记录进行分组并在 sql 中提交对每个组的查询

Question

I have a table like this:

create table product_company (
    id int PRIMARY KEY,
    productName varchar(100),
    companyName varchar(100),
    price int
);

I want to know the name of the product which it has the second rank in price in each company.

for example if company1 has three product product1=30 , product2=50 and product3=15 (the assignment shows the price of each product in this company) so product1 has the second rank in price property in company1 and I want to write a query that returns something like below:

company1 product1
company2 ...
...

I mean for every company I want to know the product that has the second rank in price within that company. I don't know how to use group by clause because group by is working fine by aggregate functions but I don't want the maximum in price.

I want to write this query with standard sql queries and clauses and without some special funcions that may not work in some DBMS

Answer 1

If you are running MySQL 8.0, you can use window function dense_rank() :

select *
from (
    select
        pc.*,
        dense_rank() over(partition by companyName order by price desc) rn
    from product_company pc
) t
where rn = 2

In earlier versions, one solution is to filter with a correlated subquery. But you have to be careful to properly handle possible top ties. This should do it:

select pc.*
from product_company pc
where (
    select count(distinct pc1.price) 
    from product_company pc1 
    where pc1.companyName = pc.companyName and pc1.price > pc.price
) = 1

Answer 2

An EXISTS with a COUNT can also be used for this

For example:

 create table product_company ( id int PRIMARY KEY AUTO_INCREMENT, productName varchar(100), companyName varchar(100), price decimal(16,2) );

 insert into product_company (productName, companyName, price) values ('product 1', 'odd org', 9) ,('product 2', 'odd org', 15) ,('product 3', 'odd org', 11) ,('product 4', 'odd org', 17) ,('product 5', 'even inc.', 18) ,('product 6', 'even inc.', 12) ,('product 7', 'even inc.', 16) ,('product 8', 'even inc.', 14) ;

 select * from product_company t where exists ( select 1 from product_company t2 where t2.companyName = t.companyName and t2.price >= t.price having count(distinct t2.price) = 2 )

\nid | productName | companyName | price\n-: |  :---------- |  :---------- |  ----: \n 2 |  product 2 | odd org | 15.00 \n 7 |  product 7 | even inc.|  16.00 \n

db<>fiddle here

And if you want to have the top 2 per company?
Then change the HAVING clause

...
   having count(distinct t2.price) <= 2
...