For you to one associations situations are easy.
If this involves someone to many or many to a lot of problems appear...
I'm not utilizing an ORM tool let's focus on a lot of reasons and i'm wondering when i wish to get data whether it's easier to reassemble someone to many relationship using multiple queries or perhaps in code..

For instance.. Getting a category Category along with a class Product...

An item table includes a collumn for category id (one category many items).

So for my full catalog is it more beneficial to complete 2 queries to obtain the groups and items i would like after which populate for every category its items List ? (It's very easy with LINQ) ..

Or i ought to call query for every category ? Like select id from products where category_id=5;

Also i dont understand how to title the functions prefer to set whether i wish to fetch sleep issues from the relationship or otherwise..

It is best to make use of the least quantity of queries easy to retrieve your computer data. Performing one query per category to load the items is called the N+1 problem, and may rapidly result in a bottleneck inside your code.

So far as things to title your techniques that specify your fetch plans, title them after exactly what the method really does, for example IncludeProducts or WithProducts.

If you wish to retrieve all groups and all sorts of their items, you may either choose all groups after which choose all items in 2 queries, or choose in a single query and group.

To make use of only one query, choose an inner join for the two tables

SELECT c.*, p.* 
FROM Category c INNER JOIN Product p ON c.CategoryId = p.CategoryId

after which construct business objects in the resulting dataset

result.GroupBy(r => r.CategoryId).Select(group =>
    new Category(/* new Category using c.* columns */)
        Products = /* new list of Products from p.* values */

But I must request - why shouldn't you be utilizing an ORM?