I actually used it, but at the time I didn't know better. I stopped using it when the default engine was still MyISAM, I heard that InnoDB improved it and made

I actually used it, but at the time I didn't know better. I stopped using it when the default engine was still MyISAM, I heard that InnoDB improved it and made it a real database. Now as you mentioned MongoDB it looks like indeed is a sister DB of MySQL. 1. Written by person (people) who did not know anything about databases and learned about them while working on them. 2. Nearly exclusive choice for hip language at the time (PHP for MySQL, NodeJS for MongoDB) 3. Performed much faster than competitive database only to turn out that it was at the cost of durability 4. New engine that supposed to improve durability (WiredTiger vs InnoDB), but even then, it still is plagued by issues caused by initial bad design.