Question

I’m learning MongoDB these days. I find that MongoDB doesn’t support join.
I just want to know why MongoDB choose to do this?

THANKS in advance..

Answer 1

1. Mongo – is not relational database and does not have physical relations and constraints.
2. Join kills scalability.

Usually denormalization replace sql join.

For example, on stackoverflow you have question and his owner, in mongodb it is normal case to denormilize owner data into question and avoid a join:

question
{
  _id,
  text,
  user_short :
  {
    id,
    full_name
  }
}

It is for sure lead to additional complexity on updates, but it give you significant performance improvements when you read the data. And for the most applications read is 95% and writes only 5% or even less.