database DDLs and implicit commit
Your question is tagged database-agnostic, so I'll provide an answer that applies to SQL Server (though I suspect that it goes for most modern platforms).

For SQL Server, the answer is no; DDL statements do not automatically COMMIT or ROLLBACK an open transaction. It normally places a schema lock on the object(s) you're touching with the DDL statement, which is held until the end of the transaction.

In fact, transactions are very useful whenever you want to combine DDL and DML statements, in order to maintain atomic business rules in your database.

Don't know about other database platforms, though.