my_solution.sql

--1st solution
select warehouse, count(warehouse) as number_of_boxes
from boxes 
group by warehouse 
union (
    select code, '0' 
    from warehouses 
    where code not in (
        select warehouse 
        from boxes
    )
);

--2nd solution
with warehouses_with_boxes (warehouse_code, number_of_boxes)
as (
    select warehouse, count(warehouse) as number_of_boxes
    from boxes
    group by warehouse
)

select warehouse_code, number_of_boxes
from warehouses_with_boxes
union (
    select code, '0'
    from warehouses
    where code not in (
        select warehouse_code
        from warehouses_with_boxes
    )
);

task.txt

Select the warehouse codes, along with the number of boxes in each warehouse. 
!!!Take into account that some warehouses are empty 
(i.e., the box count should show up as zero, instead of omitting the warehouse from the result).