Question

I wrote a “Find sids who made exactly one reservation” query with a GROUP BY and HAVING clause 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 also find it by using unique command. My friend asked me but I can’t solve it. We don’t use group by. We solve it with unique. I tried but i can’t solve it. Can you help me?

Answer 1

SELECT UNIQUE doesn’t do what you want. In Oracle it’s the same as SELECT DISTINCT. It doesn’t find the unique rows. It removes the duplicates.

You have to use GROUP BY ... HAVING ....