How to limit SQL query which uses join with Many-to-Many relationships? [duplicate] - sql

Question

I'm implementing pagination on my BD. My problem is when I want limit the SELECT statement but not the JOIN. Example, a product can got many prices:
SELECT * FROM product
LEFT JOIN price ON product.id == price.id_product
LIMIT 20
But I want to get 20 products with each one with their prices. How I can limit the statement SELECT, but not LEFT JOIN.
Example:
product price.id price.id_pruct price.price
1 1 1 50
2 2 1 30
3 3 1 40
4 1 20
5 2 30
SELECT * FROM product
LEFT JOIN price ON product.id == price.id_product
LIMIT 3
Return:
product price.id id_prodcut price
1 1 1 50
1 2 1 30
1 3 1 40
But I Want
product price.id id_prodcut price
1 1 1 50
1 2 1 30
1 3 1 40
1 4 1 20
2 5 2 30
3 . . .
Three products (limit 3)
Thanks. I hope you can help me.

Answer 1

Modify your query to limit the number of product rows before joining it to the price table. This means we want to to join the results of a query to a table, or in other words, we write a query containing a subquery:
SELECT *
FROM (
SELECT *
FROM product
ORDER BY id_product
LIMIT 3
) p
LEFT JOIN price ON p.id = price.id_product
Hope that helps.

Answer 2

I would write a subquery to get the three first products (or whatever condition you choose) like this:
SELECT id
FROM product
ORDER BY id
LIMIT 3;
Once I have that, I can select everything from the price table as long as the id is in that subquery. You can do this using a join:
SELECT p.*
FROM price p
JOIN(
SELECT id
FROM product
ORDER BY id
LIMIT 3) tmp ON tmp.id = p.product_id;
Here is an SQL Fiddle example using your sample data, and I also added a row that won't be returned so you can see that it works.