Reposting my answer to a similar question regarding SQL Server:
In the SQL world, order is not an inherent property of a set of data. Thus, you get no guarantees from your RDBMS that your data will come back in a certain order -- or even in a consistent order -- unless you query your data with an ORDER BY clause.
- So, to answer your question, MySQL sorts the records however it wants without any guarantee of consistency.
- If you are just curious about the internals of MySQL, Rolando provides an interesting answer.
- If, on the other hand, you intend to rely on this order for anything, you must specify your desired order using
ORDER BY. To do anything else is to set yourself up for unwelcome surprises.