SQL <exam>

[Jayme]

Hi,
I have an SQL exam wednesday
and I'm making some exercises in order to prepare myself.
But I don't totally understand the one that's below. :s

The correct solution was already given to me.
But it isn't very clear for me. I've added my intrepretation of what the query does, in comment.
Especially: where su1.name ='Baumgarten R.'
and not exists(
looks odd to me.

If someone understands it more than I do, please feel free to comment.
Thanks in advance.

Question:
Select the names of the suppliers who have at least as much offers as 'Baumgarten R.'(name of a supplier) does.

Using the following db scheme:
[species]
specid : a unique id for a specie
specie : name of the specie

[plants]
artcode : a unique code
name : name of the plant
specid : specie

[suppliers]
sucode : a unique supplier code
name: name of the supplier

[offers]
sucode : code of the supplier
artcode : code of the plant
price : price of the offer

**relations**
[plants] * -> 1 [specie]
[offers] * -> 1 [plants]
[suppliers] 1 -> * [offers]


----

select name from suppliers su
 where not exists ( /* select suppliers
               who < not not > have offers
               for the same species as baumgarten
               >> so only the suppliers who have at least as much offers for species where
               baumgarten also has an offer for.. 
            */
    select * from suppliers su1 
      inner join offers o1 on su1.sucode= o1.sucode
      inner join plants p1 on o1.artcode = p1.artcode
        where su1.name ='Baumgarten R.'
        and not exists(       /* suppliers name must be equal to 'baumgarten' */
                             /* and only select offers that don't go with 'baumgarten'*/
        select * from offers o2 
           inner join plants p2 on o2.artcode = p2.artcode
         where su.sucode=o2.sucode and  /* all offers that  */
               p2.specid = p1.specid ))   /* belong to <x> and where the specie appears */
                         /* with baumgarten */
 and name <> 'Baumgarten R.'