If you don’t specify an ORDER BY, then there is NO ORDER defined.
The results can be returned in an arbitrary order – and that might change over time, too.
There is no “natural order” or anything like that in a relational database (at least in all that I know of). The only way to get a reliable ordering is by explicitly specifying an ORDER BY clause.
Update: for those who still don’t believe me – here’s two excellent blog posts that illustrate this point (with code samples!) :
- Conor Cunningham (Architect on the Core SQL Server Engine team): No Seatbelt – Expecting Order without ORDER BY
- Alexander Kuznetsov: Without ORDER BY, there is no default sort order (post in the Web Archive)