Oracle SQL最后n条记录

Question

i have read tons of articles regarding last n records in Oracle SQL by using rownum functionality, but on my case it does not give me the correct rows.

I have 3 columns in my table: 1) message (varchar), mes_date (date) and mes_time (varchar2).

Inside lets say there is 3 records:

Hello world | 20-OCT-14 | 23:50
World Hello | 21-OCT-14 | 02:32
Hello Hello | 20-OCT-14 | 23:52

I want to get the last 2 records ordered by its date and time (first row the oldest, and second the newest date/time)

i am using this query:

SELECT * 
  FROM (SELECT message 
          FROM messages 
         ORDER 
            BY MES_DATE, MES_TIME DESC
       ) 
  WHERE ROWNUM <= 2 ORDER BY ROWNUM DESC;

Instead of getting row #3 as first and as second row #2 i get row #1 and then row #3

What should i do to get the older dates/times on top follow by the newest?

Answer 1

Maybe that helps:

SELECT * 
  FROM (SELECT message,
               mes_date,
               mes_time,
               ROW_NUMBER() OVER (ORDER BY TO_DATE(TO_CHAR(mes_date, 'YYYY-MM-DD') || mes_time, 'YYYY-MM-DD HH24:MI') DESC) rank
          FROM messages 
       ) 
  WHERE rank <= 2
  ORDER 
     BY rank

Answer 2

I am really sorry to disappoint - but in Oracle there's no such thing as "the last two records".

The table structure does not allocate data at the end, and does not keep a visible property of time (the only time being held is for the sole purpose of "flashback queries" - supplying results as of point in time, such as the time the query started...).

The last inserted record is not something you can query using the database.

What can you do? You can create a trigger that orders the inserted records using a sequence, and select based on it (so SELECT * from (SELECT * FROM table ORDER BY seq DESC) where rownum < 3 ) - that will assure order only if the sequence CACHE value is 1.

Notice that if the column that contains the message date does not have many events in a second, you can use that column, as the other solution suggested - eg if you have more than 2 events that arrive in a second, the query above will give you random two records, and not the actual last two.

AGAIN - Oracle will not be queryable for the last two rows inserted since its data structure do not managed orders of inserts, and the ordering you see when running "SELECT *" is independent of the actual inserts in some specific cases.

If you have any questions regarding any part of this answer - post it down here, and I'll focus on explaining it in more depth.