what does this query mean

what does this query mean

  select fname, lname  
  from Owner 
  where not exists 
      (select fname, lname 
            from Trainer)

what i understand :it returns fname and lname from table owner where these names not exsit in table traine开发者_如何转开发r ? am i right?

Find the names of people who exist in the Owner table but not in the Trainer table.

It's a pretty lame way to get a list of everyone from the owner table that doesn't exist in the trainer table.

So, if the Owner table contains

----------------
fname   | lname
----------------
clark   | kent
lois    | lane
peter   | parker

and the Trainer table contains

----------------
fname   | lname
----------------
peter   | parker
hal     | jordan

You should get a result set:

----------------
fname   | lname
----------------
clark   | kent
lois    | lane

Update

Actually, your query won't return anything if there are any records at all in the Trainer table. You should probably be using:

select fname, lname
from Owners
where not exists (
    select fname, lname 
    from trainers 
    where fname=Owners.fname 
          and lname=Owners.lname
    )

The query has two parts: the 'select' and the 'where'. Look at the where part first:

  where not exists 
      (select fname, lname from Trainer)

This where clause evaluates to 'true' if there is nothing in the Trainer table, and to 'false' if there is something there. Now look at the 'select' part:

  select fname, lname from Owners

This selects all rows from the 'Owners' table.

Putting this together: the query selects all rows from the Owner table IF (and only if) there is nothing in the Trainer table -- and absolutely nothing if there is.