How mysql order the rows with same values

Question

In my database I have some records where I am sorting by what happens to be the same value:

| col1 | timestamp              |
| row1 | 2011-07-01 00:00:00            


        

Answer 1

That depends on storage engine used. In MyISAM they'll be ordered in natural order (i.e. in order they're stored on the disk - which can be changed using ALTER TABLE ... ORDER BY command). In InnoDB they'll be ordered by PK. Other engines can have their own rules.

Answer 2

Lot's of answers already, but the bottom line answer is NO.

If you want rows returned in a particular sequence, consistently, then specify that in an ORDER BY. Without that, there absolutely NO GUARANTEE what order rows will be returned in.

I think what you may be missing is that there can be multiple expressions listed in the ORDER BY clause. And you can include expressions that are not in the SELECT list.

In your case, for example, you could use ORDER BY timestamp, id.

(Or some other columns or expressions.)

That will order the rows first on timestamp, and then any rows that have the same value for timestamp will be ordered by id, or whatever the next expression in this list is.

Answer 3

The answer is: No, the order won't be consistent. I faced the same issue and solved it by adding another column to the order section. Be sure that this column is unique for each record like 'ID' or whatever it is.

In this case, you must add the 'ID' field to your table which is unique for each record. You can assign it 'AI' (auto increment) so that you are not going to deal with the maintenance.

After adding the 'ID' column, update the last part of your query like:

SELECT mt.*
FROM my_table mt
ORDER BY mt.timestamp ASC, mt.id DESC

Answer 4

I'd advise against making that assumption. In standard SQL, anything not required by an explicit ORDER BY clause is implementation dependent.

I can't speak for MySQL, but on e.g. SQL Server, the output order for rows that are "equal" so far as the ORDER BY is concerned may vary every time the query is run - and could be influenced by practically anything (e.g. patch/service pack level of the server, workload, which pages are currently in the buffer pool, etc).

So if you need a specific order, the best thing you can do (both to guarantee it, and to document your query for future maintainers) is explicitly request the ordering you want.

Answer 5

In ORDER BY condition if the rows are same values or if you want to arrange the data by selecting ORDER BY statement. CASE : You want to ORDER BY the values of column are frequency of words. And two words in the table may have the same frequency value in the frequency occurrence column.. So in the frequency column you will have two same frequencies of two different words. So, in "select * from database_name ORDER BY frequency" you may find any of one the two words having the same frequency showing up just before its latter. And in second run the other word which was showing after the first word showing up earlier now. It depends on buffer memory,pages being in and out at the moment etc..