I've been doing BJJ for about 2 years, and I've started thinking about its role in self defense. Could it be used by itself for self defense against a single attacker without any other forms of fighting/defense? What I mean is, hypothetically, if I was randomly attacked by one unarmed person, with no clear motive, would BJJ be an effective tool without the help of any other martial arts, or would the attacker gain victory. I am of course, under the expectation that I carry out each technique accurately.
There are essentially two types of Brazilian Jiu Jitsu schools today:
1) Self-defense first, sport later.
2) Sport first, self-defense is an afterthought.
Knowing which type of BJJ school you attend will help you answer this question.
All of the Gracie schools of BJJ generally teach with the same principle: Everything in the white belt phase is for self-defense. They call this their "Combatives" curriculum. Blue belt starts going over the sport aspects. Some Gracie JJ schools won't even let you roll until after white belt.
But not all schools have this approach. They might be heavily focused from day one on the sport of BJJ, instead. Or they might want a mix from the beginning.
The reason why Gracie Jiu Jitsu always starts with a focus entirely on self-defense is because they say it will allow you to recognize when a sport adaptation won't work for self-defense later on when you start into the sport side of things. The danger in not focusing on self-defense from the beginning is that many sport techniques and strategies will only end up getting you hurt in a real fight. If you don't know how to differentiate between a sport-optimized technique and a self-defense based technique, you could be in for a surprise if you have to use it in a fight for real.
So the real question is: How do you recognize what's good for fighting vs. what's adapted for sport? And the answer to that is that you have to be taught it. Someone has to show it to you. When you're shown something that's dangerous for real fights, your teacher needs to show you why it's just for sport and not for self-defense. Doing this enough times will give you an intuitive understanding of what not to do in a real fight.
Some of what you're going to learn about fighting vs. sport is going to seem like common sense. Other stuff won't be so obvious.
Yes, you definitely can use BJJ for self-defense on its own with no other styles added to it. That's why it was created in the first place. And by the way, Gracie JJ has in its curriculum from day one strategies for dealing with strikes. Later on, you learn how to strike as well. It was influenced by boxing and kick-boxing. Gracie JJ even has a lot of classical jujitsu self-defense drills in it, including small-circle (joint locking) techniques. Some call Gracie JJ a type of MMA.
See this video to see some of the self-defense drills Gracie JJ has in its curriculum:
If you're not getting any of that from your BJJ school, it's probably a sport-based school. Nothing wrong with that, if that's what you thought it was. If you want to know the self-defense side as well, you might have to start looking into instructional videos and workshops, or start considering cross-training in another school to fill in the gaps in your understanding.
Hope that helps.