Took this class Spring2020 and it seemed like Rosario took a lot of feedback based on the previous negative reviews.

His tests were very fair and honestly on the easier side if you attended lecture and did the hw. He's very clear and has a good understanding of all the material so I made sure to attend all lectures.

He previously had large coding projects as a part of the class but this quarter instead chose to do smaller psets, which made the courseload very manageable.

He has good insights on the applications of databases due to his industry experience. Highly recommend you take this class with him

This professor definitely has changed from the previous quarters, I'm not feeling anything negative mentioned in the previous reviews in his class.

His pace is fast, and it's hard to stay engaged. Miss a lecture or two, and you have to play catchup immediately. I think the curriculum is almost too packed with a huge breadth and a lot of depth within the breadth, so our attention is spread thin. However, Rosario is a really helpful guy who genuinely cares about teaching and helps his students. The projects have kinks that need to be ironed out (project 2B at one point blocked my teammate and I for 12 hours), but I feel like I learned the most practical knowledge taking this class out of any of my upper division CS courses. He's an industry guy (works/worked at Facebook, Amazon, Google) and I think we need more of that here at UCLA. He's polarizing among my classmates but I really gained a lot from taking the course with him.

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edit: heavy emphasis on polarizing.... some of the reviews on here act like he’s the worst, a 0/10. Simply not true. If you only focus on the bad and the BS about assignments/tests (which is semi-abundant), then I understand really disliking his teaching. But if you genuinely want to learn databases, you’re gonna learn very relevant stuff w Rosario. You’ll learn some not so relevant stuff too (what’s new in cs upper divs), but this class is a nice departure from the theory-only classes that are frequent here at UCLA. Though I will say that the lack of clarity and communication on projects was frustrating, and people are justifiably mad about it. If that can get fixed for the future, then this professor is the one to take.

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TL;DR: if you’re here to absorb as much practical knowledge as you can, and can stomach some BS and perhaps a B or lower for that, then Rosario is tailored for you. But if you aren’t and/or you want a relatively easy and simple upper-div (which is totally cool, I feel ya), you shouldn’t take him. Don’t believe the hype about how bad he is. He’s been one my favorite professors so far here at UCLA, but evidently not for a lot of other people lol.

This is one of the most useful classes at UCLA. I've interned at a few well-known tech companies, and having a strong understanding of databases, SQL, and transactions is super important. Professor Rosario centers this class around preparing you for industry, because he's worked at Facebook, Google, and Amazon and he knows what we need to succeed.

There's a highly vocal group of anonymous students here who are downvoting any sort of positive feedback because they are upset about the difficulty of the final and/or their grade in the course. Notice that most of the negative reviews were written June 20-23ish, 2019, which is around the time that final exam grades and letter grades for the class were released. Don't always trust the upvotes/downvotes on each review. I'll offer a change by focusing on positive aspects, which is something I believe everyone should do rather than unleashing their hate just because they have a platform that makes them anonymous.
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First, our Piazza forum had almost 900 posts, but the professor basically answered all of them. Yup, some students received unenlightening replies or were instructed to go to office hours when they asked about things that were discussed repeatedly in lecture, but most students who politely asked reasonable questions got helpful answers -- within MINUTES -- from the professor himself. If a professor did not care about his students, would he answer practically all of the students' concerns within 1-10 minutes when he could just assign the task to the TAs like other "oh so caring" or in other words "gives lots of A's" professors do? Do other professors stay up until midnight the day before the 8am final exam to answer all last-minute questions from students, like this professor does?
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Second, the professor is actually really nice and friendly in person. During the break and after class, I've seen students approach him and ask a few questions, and the professor would typically respond with a friendly smile and often laugh lightheartedly. If good questions came up during the break, he would announce them and clarify things to the entire class as soon as the class resumed. He also held extra office hours during Weeks 2-10 upon requests from students, and he held extra office hours during Finals Week that spanned several hours as well.
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Third, his projects were pretty interesting and sometimes fun although they were difficult. They were much more practical than other professors' projects, some of which were last updated a decade ago. He also gave extra credit on the projects, and they weren't too difficult as long as you planned ahead.
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Fourth, the professor is truly loyal to UCLA and gave several inspirational pep talks throughout the quarter. He mentioned how he did horribly in some classes here at UCLA but that it did not affect his professional career. He name-dropped several prestigious jobs he held and companies he worked for in the past to illustrate that grades don't dictate our future.
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All of my friends like this professor, and so does a good chunk of the class, which is evident because a lot of students continued to go to lecture even though it was at 8am. Students often laughed at the professor's memes and jokes, and generally had feel-good vibes as the class came to a close. We applauded the professor not once but twice at the end of the last lecture to express our appreciation. The emotional, vocal minority here on Bruinwalk does not reflect all 200+ students in the class. However, I do understand these students' struggles because this class definitely pushed us hard in terms of assignment workload, exam difficulty, and denseness of class material. Let's just remember to be nice to others even when we're anonymous so that we can collectively strengthen our community, do good things out there, and be regarded with greater respect from those outside the Bruin family.

He is a genuinely helpful professor, who is always trying his best to help us learn and improve.

This was honestly one of the weirdest classes I've ever taken. The class started out extremely easy...the first project was a joke (easier than a CS31 project) and the midterm was very straightforward and fair.
However, once we moved on from SQL and started learning about more theory-intensive subjects, the entire class went completely downhill. Our second project was an absolute shit show. For some people, just RUNNING the project took 10+ hours since we were dealing with big data...and while we did learn very valuable information about Spark, I feel like there would've been a more efficient way to teach us this material. Additionally, one of the things that annoyed me the most was that he would not give us concrete methods to approach a problem...he would say things like "oh it won't show up on exams don't worry about it" which was extremely frustrating as a student. His homework assignments were horrible...they made no sense and needed a lot of clarification (you should've seen our Piazza LMAO). Our final was also the hardest test I've taken at UCLA and half the material was from like 1 or 2 slides from his powerpoints.
Overall, this class was okay. I learned very valuable information (esp SQL) and would definitely recommend to take it before you graduate. However, this class with Rosario was just extremely frustrating because of the lack of clarification in a lot of his projects/assignments. Don't get me wrong—Rosario was a great lecturer and his slides were very helpful (I really loved attending his lectures and he was very responsive on Piazza). I just didn't like his assignments and the overall structure of the class.

Bad!!!!!!!!

---TLDR---
In my opinion, Rosario's intentions aren't expressly bad, but he often acts in ways that don't help anyone. He frequently characterizes things he doesn't seem to really understand as 'unnecessary', 'impractical', and 'not relevant to industry'. He frequently tries to act cool - and sometimes is cool - but often simply acts in unreasonable ways.
Most of the things I wanted to say are already written on bruinwalk, but here's some more stories.
---Stories---
When discussing some dependency stuff, topological sort came up and he made a mistake in explaining the material. Mistakes are honestly fine, but what isn't fine is that when a student questioned it during lecture, he quickly disregarded it and became increasingly upset as more students realized he didn't know what he was talking about. He then proceeded to yell at the student, rush through the slides, and then, near the end, reference his meme of a crying baby to state that everyone is picking on him (the "picking on" text was underneath a picture of a crying baby on the slides). I guess people have their off-days and its not a big deal, but he refused to ever clarify the material, and then put a question on the final regarding it. I feel that's not very cool of him.
I might give him a pass for this whole topological sort fiasco if it weren't for the fact that for the midterm, he said "we will only make passing reference to them" regarding recursive and window queries! And then he put them both on the midterm! (Rosario often says that he fully specifies what is on his exams - this is not true, do not believe it.)
Moreover, he said in office hours once that triggers and constraints will be extremely important for the test, but they only came up as a tangentially for one out of 200 points!
Also, he said that B+-Trees were not to be tested (because he severely messed up teaching the topic, and was unable to explain the difference between a B+-Tree and a B-Tree during lecture, and even the homework on the subject was broken because the insertion-style was ambiguous and inconsistent lol), but then it was on the final.
Calculating the value for hyperloglog was on the final. Calculators were not allowed, so we were expected to compute a*m^2*Z where Z = 1/sum_{j=0}^m 1/2^{c_j} and then round only at the last step? Honestly I don't know what that formula means, I just copied it from the slides right now. I left that question blank on the final because I didnt have the formula lol.
Oh also, imo, his slides are very unclear. just fyi. I relied on other school's notes the entire quarter tbh.
---Reddit information---
Before you take a class under this professor, be aware of this (seems that Rosario reported a student to the police regarding a discussion on reddit?) https://removeddit.com/r/Professors/comments/bzyco2/the_attendance_dilemma/ercowpa/?context=3
---Final distribution info---
For anyone looking for stats regarding the final, a 100/200 is a B, mean/median is 42%, high is 72%

So I signed up the account just for this professor, I got a bad grade in this class but don't get me wrong I get B+ for other cs courses as well but this grade is really not what i was expecting.
Projects: To be fair, the projects are interesting and I actually enjoy doing them. However the timeline RRR set up was ridiculous. For project 1A we have two weeks just to load the data, and for the same amount of time we are asked to do a much harder task.
Exam: Midterm is fair, we are tested on SQL queries and related stuff, fully expected. BUT THE FINAL, it was a disaster. For the final RRR tried really hard to make the questions in an eggert style but failed. We were not given any practice tests and the content was just randomly picked from the lecture slides... the average was around 84/200. And one of my friends just told me for the final he "expected" us to do better (maybe because he thought the questions were easy?) but as you can see from our average they weren't. So he just made the average of final a C. Well, I don't know what else to say.
Overall the class content is fine and interesting, but try not take it with RRR.