LINQ Query Variable passed to Another LINQ Query - linq

Question

What is the difference between the query in this post:
Save LINQ Query As Variable To Call Another LINQ Query
var parentLoc = (from a in db.PartsLocations
where a.LocationName == aContainer
select a.ParentLocation);
var locations = (from b in db.PartsLocations
where b.LocationID == parentLoc
select b).ToList();
to the following example from this post.
Dim persVogel = From p In db.People
Where p.LastName = "Vogel"
Select p
Dim persVogelPHVIS = From pp In persVogel
Where pp.Company.Name = "PHVIS"
Select pp
Both of these have declared 2 queries and use the first query variable into second query.
What is the reason to use Single() in the first example but was not used in visualstudiomagazine.com article? Thanks

Answer 1

The two samples are fundamentally different.
First Sample
I think you pasted this one incorrectly...from the referenced question, the query should be:
var parentLoc = (from a in db.PartsLocations
where a.LocationName == aContainer
select a.ParentLocation).Single();
var locations = (from b in db.PartsLocations
where b.LocationID == parentLoc
select b).ToList();
(I am going to assume that LocationID and ParentLocation are typed as int.)
In this sample parentLoc is an int - a single instance of a ParentLocation value, obtained from the PartsLocations table. So what you get is an int.
The second linq statement sources its records also from the PartsLocations table. It uses parentLoc to identify records within that table (e.g. where b.LocationID == parentLoc). What you get at the end is a set of PartsLocations records.
The .Single() call is made because you want to compare the result to LocationID in the second statement, and cannot compare int to IEnumerable<int>.
Second Sample
Dim persVogel = From p In db.People
Where p.LastName = "Vogel"
Select p
Dim persVogelPHVIS = From pp In persVogel
Where pp.Company.Name = "PHVIS"
Select pp
In the second sample, persVogel is a subset of records from the People table (specifically, the subset of people with LastName == "Vogel") - so what you get is a set of People records.
The second linq statement is based on this subset of records (From pp In persVogel) and further filters them down to records where pp.Company.Name = "PHVIS". What you get is still a set of People records.
These two statements could easily be compressed into one single statement:
Dim persVogelPHVIS = From p In db.People
Where p.LastName = "Vogel"
AndAlso p.Company.Name = "PHVIS"
Select p
You will still get a set of People records at the end.