Entity Queries in Unity ECS - Deep Dive into Finding Data - Unity DOTS Tutorial [ECS Ver. 0.17] 

❗❗ *Caution:* This video was made using an older version of Unity ECS. While the core concepts remain the same, some of the API and workflows have changed as of ECS version 1.0. I would recommend checking out my ECS 1.0 tutorial video for a good overview of the latest standards for Unity’s DOTS: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-IO6_6Y_YUdE.html Once again, the theory behind the concepts of this video are still relevant, however the API has changed. Stay tuned for further updated videos on this subject. Please let me know if you have any questions either here or in our Discord community: tmg.dev/Discord

Just to expand on "ref" and "in" keywords. They are used because ECS works with structs which are value types (classes are reference types). Any time you pass a struct as a parameter or make a new variable of it, without using "ref" or "in", you are actually creating a copy of it. Any changes made to that copy won't affect the original reference. And every copy will consume space in memory. Technically, they are more efficient when used correctly and sparingly. However they can very quickly become highly inefficient if abused. "ref" and "in" in short just tells the compiler we want to work with a reference of a struct (treating it similarly to a class). "ref" means a reference to the struct with the ability to modify it. "in" means a reference to a struct without the ability to modify it. You could technically drop "in" and just pass the struct as a parameter creating a new copy of it (I don't think Unity allows this, I'm just using it as an example). This is technically faster and you won't risk modifying the original reference. However, once again you are working with a copy. If you have thousands of systems with thousands of entities you can see how quickly that will become a problem. The reason we use structs and not classes is because classes are objects which can only exist in memory (Slow) whereas structs can work on the the stack (Fast).

Awesome explanation, clears out many of my misconception cause I'm still new to ECS!

Thanks for the incredible content

Solid info. Now i will conquer the Universe with these powers...

I've been following along with your ECS series and hit at a roadblock: in your example, if you'd wanted to reference different groups of soldiers, how would you go about tagging them, then querying each group? I've watched your video on SharedDataComponent and it seems like the closest solution, but doesn't actually work with queries.