Question

Look at this DB schema:

The users can order products. For any order I add a new record in the orders table and a new record in orders-products N:M table for any single product ordered by the user (and fill pieces field). But, when a user creating an order, I need to show the list of all products and fill the pieces field with the quantity ordered for any product.

Usually, to do it, I use two queries.
The first query get all products in the products table:

SELECT * FROM products;

The second query gets the products ordered in the orders-products table filtering them by orders-idorder FK:

SELECT orders_idorder AS idorder, products_idproduct AS idproduct, pieces FROM orders_products WHERE orders_idorder=1;

Then I merge the results in a single array and use it to display the complete list of products with the pieces ordered for each one. The final result is something like this:

+---------+-----------+--------------+-------+--------+
| idorder | idproduct | description  | price | pieces |
+---------+-----------+--------------+-------+--------+
|       1 |         1 | Product 1    | 10.20 |      2 |
|       1 |         2 | Product 22   | 11.00 |   NULL |
|       1 |         3 | Product 333  | 19.22 |   NULL |
|       1 |         4 | Product 4444 |  9.20 |   NULL |
+---------+-----------+--------------+-------+--------+

Note: In the above example there are 4 records in the products table and just 1 record in the orders-products table (it has idorder=1, idproduct=1 and pieces=2).
-> Here you can find the SQL Dump to test the queries.

Merging arrays built from 2 queries is the best way?
I can do it with a single query?
What do you think about the performance?

Answer 1

Generally letting the databse optimize the merge will be better than what you can do in code. The db is usually better at sorting too.

How you structure the query depends on your desired result set. If you want all products regardless of whether they appear in the orders table then you’d use an OUTER JOIN otherwise an INNER JOIN would filter out products that have never been ordered.

If you give us your desired results for some sample data we might be able to help you with the query, but give it a shot yourself first.

set @searchOrderId = 1;
select ifnull(op.orders_idorder, @searchOrderId) AS idOrder,
    p.idproduct,
    p.description,
    p.price,
    op.pieces
from products p
left join orders_products op on op.products_idproduct = p.idproduct
where ifnull(op.orders_idorder, @searchOrderId) = @searchOrderId ;

SQL Fiddle I was playing with to test it.