MySQL is the first Database I have experience with. In my mind, it is the industry standard Relational Database. To call it ubiquitous is an understatement. MySQL is very easy to get set up and running and is highly performant. Unfortunately, MySQL has a shaky past due to Oracle's acquisition of Sun Microsystems back in 2009. Thanks to Oracle's reputation for being unfriendly towards Open Source software, a fork of MySQL named MariaDB was created and has splintered MySQL development for the past decade. Given the two, I prefer to use MariaDB both out of principle, and because I find MariaDB a better Database. However, this fractured ecosystem often leads me to simply choose another Database, namely PostgreSQL or MongoDB.
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