Hive installation guide says that Hive can be applied to RDBMS, my question is, sounds like Hive can exist without Hadoop, right? It's an independent HQL engineer that could work with any data source?
You can run Hive in local mode to use it without Hadoop for debugging purposes. See below url
Hive provided JDBC driver to query hive like JDBC, however if you are planning to run Hive queries on production system, you need Hadoop infrastructure to be available. Hive queries eventually converts into map-reduce jobs and HDFS is used as data storage for Hive tables.