What's the most efficient way to check if a record exists in Oracle?

Question

A)

select decode(count(*), 0, \'N\', \'Y\') rec_exists
from (select \'X\'
      from dual
      where exists (select \'X\'
                    from sales
           


        

Answer 1

select CASE 
when exists (SELECT U.USERID,U.USERNAME,U.PASSWORDHASH
FROM  TBLUSERS U WHERE U.USERID =U.USERID 
AND U.PASSWORDHASH=U.PASSWORDHASH) 
then 'OLD PASSWORD EXISTS' 
else 'OLD PASSWORD NOT EXISTS' 
end as OUTPUT
from DUAL;