All of them are good certification you can choose any one of them, Certification is justifying a way for professionals like IT employees to validate their knowledge and obtain credibility that will lead a candidate to better opportunities.
Oracle PL/SQL Developer Certified Associate Certification
PL/SQL is the programming language supported for writing Stored Procedures in Oracle. PL/SQL expertise is often demanded by employers, although I personally recommend avoiding stored procedures if possible.
Oracle Database SQL Certified Associate Certification
Presumably this is supposed to indicate that you have mastered the SQL language as it applies to Oracle databases.
MySQL 5.6 Developer Certification
MySQL is a former open-source DBMS. It is not overly compatible with Oracle (less so than PostgreSQL), but an awfully large part of the Internet has been built on MySQL. Because a lot of people felt uncomfortable about Oracle having control of such a sensitive asset, MySQL was forked into the open-source MariaDB. At the moment, the two products are pretty much interchangeable, but time will tell. Oracle also bought OpenOffice, but that caused a fork producing LibreOffice. I think OpenOffice has since been donated to the Apache Foundation.
Phil Niksen wrote:
Also are there any study guides for these?
Oracle has certificate training courses, usually expensive ones, and if I remember aright, some of them are on-line courses, although I did Oracle Financials at one of their regional facilities. If your budget is more limited, a quick Internet search should yield a wealth of other alternatives. And, of course there are books, including books from Oracle Press.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.