Last record chosen in Oracle Query

I need to only retrieve the last record when this Oracle 9.2 query executes.

DI  2016-09-26  168398    063848979   //SRC RMS


I would also like another Oracle query that only retrieves the first record.

NULL      DI  2016-09-16  174261    063848979   //SRC RMS             FB BRAMPTON TO

Right now,  the query will retrieve 3 records for this problem when it executes.
The 1993237 will be a field once I get it to work.
This Oracle query executes inside a SQL query.




SELECT (CASE WHEN TO_CHAR(aa.NOTES)  IS NOT NULL  THEN  TO_CHAR(aa.NOTES)  ELSE TO_CHAR(a.PTEXT) END) as MyNote
FROM            GPCOMP1.GPPROB a, GPCOMP1.GPPROBNOTES aa
WHERE        a.PROBLEM_ID = aa.PROB_ID AND (a.PROBLEM_ID = 1993237)
ORDER BY a.MODIFIED_ON DESC