If the goal is to learn and to improve your CV, I would install all these systems.
But definitely include Oracle: it's pretty much the gold standard of database systems, and there is always well-paid work. Oracle is to be avoided if you have to pay for the system (it's ridiculously overpriced) but for a developer it is a good skill to have. There is a free developer install.
If you also install the free/opensource systems PostgreSQL, MySQL and SQLite, you can learn how to write programs that talk to the different systems in the same way (=with the same/similar SQL statements). This way you'll learn the differences and specific strengths of each system.
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