Which way will get high performance while selecting many data IQueryable Vs For loop (Using Entity Frame Work) - linq

Question

I am trying to get a list from the database containing two or more lists inside that list.(using .net core, entity framework).Assume I have two table call header and details table.
Header Table
Detail Table
And I want the result like this:
{
"data":[
{
"Country":"Singapore",
"Hospital_List":[
{
"Hospital_Name":"SG Host A"
},
{
"Hospital_Name":"SG Host A"
}
]
},
{
}
]
}
I only know two ways to get the result like this,First Way, select Country list data with blank Hospital list as List,then for loop that list to select related Hospital list from db again.
And Second Way,select Country list data with blank Hospital list as IQueryable List,and then select related Hospital list via jointing with Hospital Table.So my question is
Which way should i used to get higher performance? And Is any other way?
Please remember there has a lot of field and data in my real table.

Answer 1

For loop give give you the lowest perfomance, because you will create SQL query for each iteration. Instead of this, try following solution:
from hospital in hospitals
group hospital by hospital.CID into gh
join country in countries
on gh.FirstOrDefault().CID equals country.CID
select new
{
Country = country.Country,
Hospital_List = from h in gh select h
}
EDITED:
And if your model created right you can use this code:
from hospital in hospitals
join country in countries
on hospital.Country equals country
group hospital by hospital.CID into gh
select new
{
Country = from h in gh select h.Country.Country,
Hospital_List = from h in gh select h
}