Question

I write “Find sids who made exactly one reservation”query with Using group by having like this:

SELECT R.sid
FROM   Sailors S,
       Reserve R
WHERE  R.sid = S.sid
GROUP  BY R.sid
HAVING Count(R.sid) = 1 

But I want to alsa find it by using (not exists) command.I write it

SELECT S.sid
FROM   Sailors S
WHERE  NOT EXISTS (SELECT R.sid
                   FROM   Reserve R
                   WHERE  Count(R.sid) <> 1) 

But it has errors.How can I write it by using not exists command.

Answer 1

If Reserve has a primary key rid, you can write it like so:

Select
  s.sid
From
  Sailors s
    Inner Join
  Reserve r
    On s.sid = r.sid
Where
  Not Exists (
    Select
      'x'
    From 
      Reserve r2
    Where
      r2.sid = s.sid And
      r2.rid != r.rid
  );

Note, in this case you don’t even need to reference the Sailors table:

Select
  r.sid
From
  Reserve r
Where
  Not Exists (
    Select
      'x'
    From 
      Reserve r2
    Where
      r2.sid = r.sid And
      r2.rid != r.rid
  );

http://sqlfiddle.com/#!3/cce6c/2