MySQL LEFT JOIN, GROUP BY and ORDER BY not working as required

Question

I have a table

\'products\' => (\'product_id\', \'name\', \'description\') 

and a table

\'product_price\' => (\'pro         


        

Answer 1

This will give you the last updated price:

select
  p.*, pp.price
from
  products p,
  -- left join this if products may not have an entry in prodcuts_price
  -- and you would like to see a null price with the product
  join 
  (
      select 
        product_price_id, 
        max(date_updated) 
      from products_price
      group by product_price_id
   ) as pp_max
    on p.product_id = pp.product_id
  join products_price pp on 
    pp_max.prodcuts_price_id = pp.products_price_id

Answer 2

It appears that it is impossible to use an ORDER BY on a GROUP BY summarisation. My fundamental logic is flawed. I will need to run the following subquery.

SELECT `p`.*, `pp`.`price` FROM `products` `p` 
LEFT JOIN (
    SELECT `price` FROM `product_price` ORDER BY `date_updated` DESC
) `pp` 
ON `p`.`product_id` = `pp`.`product_id`
GROUP BY `p`.`product_id`;

This will take a performance hit but as it is the same subquery for each row it shouldn't be too bad.

Answer 3

You need to set aliases properly I think and also set what you are joining on:

SELECT p.*, pp.price 
FROM products AS p 
LEFT JOIN product_price AS pp
ON pp.product_id = p.product_id 
GROUP BY p.product_id 
ORDER BY pp.date_updated DESC

Answer 4

Mysqlism:

SELECT p.*, MAX(pp.date_updated), pp.price 
FROM products p 
LEFT JOIN product_price pp ON pp.product_id = p.product_id
GROUP BY p.product_id 

Will work on some RDBMS:

SELECT p.*, pp.date_updated, pp.price 
FROM products p 
LEFT JOIN product_price pp ON pp.product_id = p.product_id
WHERE (p.product_id, pp.date_updated) 

   in (select product_id, max(date_updated) 
       from product_price 
       group by product_id)

Will work on most RDBMS:

SELECT p.*, pp.date_updated, pp.price 
FROM products p 
LEFT JOIN product_price pp ON pp.product_id = p.product_id

WHERE EXISTS
(
    select null -- inspired by Linq-to-SQL style :-)
    from product_price 

    WHERE product_id = p.product_id 

    group by product_id

    HAVING max(date_updated) = pp.date_updated
)

Will work on all RDBMS:

SELECT p.*, pp.date_updated, pp.price 
FROM products p 
LEFT JOIN product_price pp ON pp.product_id = p.product_id

LEFT JOIN 
(
    select product_id, max(date_updated) as recent 
    from product_price 
    group by product_id
) AS latest 
ON latest.product_id = p.product_id AND latest.recent = pp.date_updated

And if nate c's code intent is to just get one row from product_price, no need to table-derive (i.e. join (select product_price_id, max(date_updated) from products_price) as pp_max), he might as well just simplify(i.e. no need to use the product_price_id surrogate primary key) it like the following:

SELECT p.*, pp.date_updated, pp.price 
FROM products p 
LEFT JOIN product_price pp ON pp.product_id = p.product_id
WHERE pp.date_updated = (select max(date_updated) from product_price)