This may strongly depend on the database. In Oracle, for example, you can start the transaction in serializable mode (or read-only mode for that matter, though JDBC does not directly support read-only transactions) and all your selects will return data as they existed at the time the transaction started. This may be important in regard to the consistency of data being read.
In other databases the transaction (and its isolation level) might affect the results in different ways.
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