> You're not limited to making updates in a transaction; you can also issue complex
> queries to extract data, and possibly use that information to build up more updates
> and queries; while the transaction is active, you are guaranteed that no one else can
> make changes while you are in the middle of your work. For further reading on
> transactions, refer to the documentation provided by your database server.
This only holds true if you specifically do "SELECT .... FOR UPDATE".
Without the "FOR UPDATE" part, when two transactions run at the same time, the second transaction could change a value AFTER the first transaction read it, but BEFORE the first transaction used it for updates.
Without the "FOR UPDATE" part you are absolutely NOT GUARANTEED that no one else can make changes while you are in the middle of your work.
