Question

1) Subquery is a query within a query and as such must start with a FROM clause and end with SELECT or GROUP BY clause. But I’m puzzled why don’t we also consider as a subquery those nested statements that only have a FROM clause ( thus they don’t end with SELECT or GROUP BY ).

The behavior of from c_2 in collection_2 is very similar to the behaviour of a subquery, since it enumerates the entire collection_2 for every c_1 element. Since their behaviours are remarkably similar, why aren’t statements such as from c_2 in collection_2 also considered as subqueries ( you could argue that a subquery returns a result, but same argument could also be said of from c_2 in collection_2, since it does get converted into a call to SelectMany )?

    var query = from c_1  in collection_1
                from c_2  in collection_2 
                select ...

thank you

Answer 1

but same argument could also be said of from c_2 in collection_2, since it does get converted into a call to SelectMany

Not really. That one call doesn’t get converted into SelectMany. It’s the fact that there are two from clauses, in combination (with just a single select) that causes it to be converted into a SelectMany call, so the whole thing is a single query in that case. If each from matched up with a single select then it would be a subquery.