Ask HN: Databases and licenses

I have worked many years as an employee developing software projects, and as an employee I never cared about licenses. Now that I have an startup, I would like to know if it is ok for me to use a Community Edition of a database (or other software/libraries), for a project which is not open source (a project for a client, for instance). The most common case would be MySQL, but I'm also looking at databases like VoltDB, MariaDB, Drizzle. I don't know what do licenses such as GPL3 allow and forbid me to do. Perhaps I don't have to worry as long as I don't modify the database source code, and if I do so I must provide source code of my modifications?