See the comments from @Richard-Deeming for the solution to your immediate problem.
You need to review how you are querying your data overall - if you have a food item that has no imports or exports, or only has imports, or only has exports, then that food item will never appear in your query because you are using INNER joins to your sub-queries.
For example if you insert the following data
insert into @foods (food_id, foodname, foodownername, balance) values
(1, 'Rice','Joe',200),
(2, 'Flour','Jane',100),
(3, 'Tea','Jack',800),
(4, 'Oil', 'Jennifer',1000);
insert into @imports (food_id, quantity) values
(1, 150),
(2, 250),
(3, 350);
insert into @exports (food_id, quantity) values
(1, 175),
(2, 275),
(4, 555);
Then your query will return
food_id foodname foodownername balance import_quantity export_quantity
1 Rice Joe 200 150 175
2 Flour Jane 100 250 275
Whereas you probably wanted to see
food_id foodname foodownername balance import_quantity export_quantity
1 Rice Joe 200 150 175
2 Flour Jane 100 250 275
3 Tea Jack 800 350 0
4 Oil Jennifer 1000 0 555
There is also no need for using sub-queries if you restructure your query differently, and there is every chance it will perform better without the sub-queries. E.g.
SELECT f.food_id,f.foodname,f.foodownername,f.balance
,ISNULL(SUM(i.quantity),0) as import_quantity
,ISNULL(SUM(e.quantity),0) as export_quantity
FROM foods f
LEFT OUTER JOIN imports i ON i.food_id = f.food_id
LEFT OUTER JOIN exports e on e.food_id = f.food_id
GROUP BY f.food_id,f.foodname,f.foodownername,f.balance
(Caveat: This is MSSQL not MySQL so you may need to change the ISNULL function to the MySQL equivalent)