Is it true that to learn Java thoroughly one should know Oracle?
I hope not. I never intend to use Oracle myself. On the other hand, it could be that most of the job openings for java programmers are to interface to Oracle. But wouldn't interfaces such as JDBC, JPA, Hibernate, Eclipselink, or .... be a sufficient knowledge to acquire if a programmer in planning to persist application data? Wouldn't all of the skills of System Administrator, Database Administrator, and Data Analyst be overkill for a typical entry level Java application developer?
No need to learn oracle. But it is suggested to know SQL not Oracle. Even, if you are working on core java or J2ME, no need to learn SQL. Learn java, you automatically know the depth of Database importance.
(SCJP5.0, IBM Rational Solution Designer, IBM Rational Architect, IBM RUP Designer)
Oracle is just one particular brand of database which is used a lot in big companies. But it is certainly not necessary to "learn Oracle" or even learn anything about databases before you learn Java.
When you are getting a job as a Java programmer, then sooner or later you will most likely have to work with a relational database and SQL, so at some time you will need to learn how databases and SQL work. But it doesn't necessarily have to be Oracle.