Performance for using 2 where clauses in LINQ

First Where returns IQueryable<T> so there will be no performance difference.

The first .Where() clause will still return an IQueryable<T>. As long as you are operating on an IQueryable<T> it will continue building up the SQL query and execute it when the collection needs to be brought into memory (eg: as @anaximander stated when used in a foreach loop or ToList() operation.

Therefore:

SchoolContext.Students.Where(s => s.Name == "Foo").Where(s => s.Id == 1);

Still translates into:

SELECT *
FROM Students
WHERE Name = 'Foo' AND Id = 1

Whilst the below 2 statements will translate into the same query:

SchoolContext.Students.Where(s => s.Name == "Foo").Where(s => s.Id == 1);
SchoolContext.Students.Where(s => s.Name == "Foo" && s.Id == 1);

The two should produce the same SQL; IQueryable is smart in that it doesn't actually evaluate until it needs to. The second .Where() should add to the first, and then whenever you use .ToList(), .Count(), foreach or anything that needs to know what's in the IQueryable, it'll generate the SQL, query the database and give you the result.

No, it will be the same query.

You can compose a more and more complex query by fluently chaining the IQueryables returned by each Linq operation. The statement is not generated and sent to the server until you evaluate the results.

You can check that the actual query generated by debugging and hovering over the query or doing a ToTraceString() or using a tool like SQLProfiler to monitor the database server.