SQL query to select columns from subquery

Question

I am trying to start using subqueries which to this point I haven't needed to. If I have these tables:

Books
--Id (Primary Key)
--Title (string)
--GenreId (Foreign Key)

and

Genre
--Id (Primary Key)
--Name (string)

I know that if I want to retrieve Title and Name, then I can run this query:

select b.Title, g.Name from Books b
join Genre g on b.GenreId=g.Id

What I am trying to do is understand the difference between that and this:

select b.Title from Books b
join (
    select g.Id, g.Name from Genre g
)
as ge
on b.GenreId=ge.Id
go

I want to know why the second query only gives me the Title column from Books and not the Id or Name columns from Genre? My question really is how can I turn the first query into a subquery and retrieve the same results? I want both Title from Books, and Name from Genre.

Answer 1

The second query only gives you the title because that is all you asked for.

The outermost select describes the result set you are getting. Other select s are used for the processing logic but don't affect the columns being returned.

Answer 2

尝试这个。

  select G.name, B.title from Books B left join Genre G On G.ID = B.GenreId