SQL: What is the default Order By of queries?

There is no such order present. Taken from http://forums.mysql.com/read.php?21,239471,239688#msg-239688

• Do not depend on order when ORDER BY is missing.

• Always specify ORDER BY if you want a particular order -- in some situations the engine can eliminate the ORDER BY because of how it does some other step.

• GROUP BY forces ORDER BY. (This is a violation of the standard. It can be avoided by using ORDER BY NULL.)

SELECT * FROM tbl -- this will do a "table scan". If the table has never had any DELETEs/REPLACEs/UPDATEs, the records will happen to be in the insertion order, hence what you observed.

If you had done the same statement with an InnoDB table, they would have been delivered in PRIMARY KEY order, not INSERT order. Again, this is an artifact of the underlying implementation, not something to depend on.

There's none. Depending on what you query and how your query was optimised, you can get any order. There's even no guarantee that two queries which look the same will return results in the same order: if you don't specify it, you cannot rely on it.

I've found SQL Server to be almost random in its default order (depending on age and complexity of the data), which is good as it forces you to specify all ordering.

(I vaguely remember Oracle being similar to SQL Server in this respect.)

MySQL by default seems to order by the record structure on disk, (which can include out-of-sequence entries due to deletions and optimisations) but it often initially fools developers into not bother using order-by clauses because the data appears to default to primary-key ordering, which is not the case!

I was surprised to discovere today, that MySQL 5.6 and 4.1 implicitly sub-order records which have been sorted on a column with a limited resolution in the opposite direction. Some of my results have identical sort-values and the overall order is unpredictable. e.g. in my case it was a sorted DESC by a datetime column and some of the entries were in the same second so they couldn't be explicitly ordered. On MySQL 5.6 they select in one order (the order of insertion), but in 4.1 they select backwards! This led to a very annoying deployment bug.

I have't found documentation on this change, but found notes on on implicit group order in MySQL:

By default, MySQL sorts all GROUP BY col1, col2, ... queries as if you specified ORDER BY col1, col2, ... in the query as well.

However:

Relying on implicit GROUP BY sorting in MySQL 5.5 is deprecated. To achieve a specific sort order of grouped results, it is preferable to use an explicit ORDER BY clause.

So in agreement with the other answers - never rely on default or implicit ordering in any database.