MYSQL: how to get one row from multiple rows with same column id value

Question

i have two tables: product and productVariation: product variation table contains multiple entries for the same product but with different attributes.

example: product: id name 1 top 2 bottom

id	name
1	top
2	bottom

productVariation table:

id	product_id	color_id	size_id
1	1	4	7
2	1	5	7
3	1	8	7
4	1	7	10

so for the above tables, when I am querying that data for the products list to display, let's say the user wants data for size_id 7 then we have 3 possibilities (3 different colors) but on the products list page, I just want to show one entry for this product. My question is how can I just get one entry based on the product_id from the product variation table. I already wrote a query that is filtering the data based on color and size but I get multiple entries for the same product, I just want one of these rows.

Thank you: :)

Answer 1

I'm guessing row_number could be useful here.
If your version of MySql or MariaDB supports it.

SELECT product_id, product_name, color_id, size_id
FROM
(
    SELECT 
      pvar.product_id
    , product.name AS product_name
    , pvar.color_id
    , pvar.size_id
    , row_number() over (partition by pvar.product_id order by pvar.id DESC) AS rn
    FROM product AS product
    JOIN productVariation AS pvar
      ON pvar.product_id = product.id
    WHERE pvar.product_id IN (1)
      AND pvar.size_id = 7
) q
WHERE rn = 1

Or maybe just LIMIT the resultset.

SELECT 
  pvar.product_id
, product.name AS product_name
, pvar.color_id
, pvar.size_id
FROM product AS product
JOIN productVariation AS pvar
  ON pvar.product_id = product.id
WHERE pvar.product_id IN (1) 
  AND pvar.size_id = 7
ORDER BY pvar.product_id, pvar.id DESC
LIMIT 1

Answer 2

You could aggregate your multiple colors and group by the size, taking the min or max as the one to pick. Doing so in a lateral join would look like

select * 
from products p
left join lateral(
  select size_id, max(color_id) color_id
  from productvariations v
  where v.product_id = p.id
  group by size_id
)pv on true
where pv.size_id = 7;