MongoDB for Developers with Python Transcripts
Chapter: MongoDB's shell and native query syntax
Lecture: Advanced queries
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Now, let's use Robomongo, our shiny new shell
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that I contend is better than just the cli one,
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let's use it to explore some more advanced query filtering and sorting options.
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So here's just a blank find showing me all the records,
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how many are there in the database, there's 271 thousand books,
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so this is the same database we've been playing with for a while now.
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So let's ask some questions about the ratings.
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So we're going to go into the ratings array
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which contains a bunch of objects, which have values,
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so I want to say how many of them have the value nine,
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so what's that actually answering— what question is that answering
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that is answering how many books have been rated
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at some point by somebody with a nine,
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how about with ten— a little bit more,
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so there are some books that were really, really popular
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people loved them, this is a 1 to 10 type of scale,
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I think it might also include zero.
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So that's great, this is our prototypical json object here.
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However, what if I want to say show me all the books
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that have a moderately high rating, what does that mean,
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let's say it has an eight, a nine or a ten as a rating,
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how do I express that as a prototype? You can't do it,
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and so that's why MongoDB has something slightly more complex
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and nuanced than just straight comparison, right,
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so this is like an equality query, so instead of putting a value here
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we can put a little sub search document here
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and into this, we can say I'd like to apply an operator instead of an exact match,
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so the operator might be greater than operator >,
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so the way you know it's an operator is the dollar
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and gte greater than or equal to is going to be the thing
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and then we're going to put the value of eight,
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so show me the books that have a rating of eight or above,
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tell me how many there are because we're doing a count,
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so let's run that, look at that 98 thousand books
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have a rating of eight, a nine or a ten.
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Does it mean their average rating as eight, nine or ten,
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that means somebody somewhere has rated it eight, nine or ten.
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So we also have things like greater than,
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without the equal, just flat up greater than so that's nine or ten right there,
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so we have a number of these operators,
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greater than, greater than or equal to, and so on.
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Another one that's really interesting is in,
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this is super important for really powerful queries,
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so when we have documents that contain sub arrays of other documents
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you can think of those as basically being pre joined
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but when you normalize those, that are not contained within each other,
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then you need a way to still go back and say
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basically do the join, and this in operator is the key to making that happen,
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this is not really what's happening here, because this is a sub document,
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but it's the operator that's involved, so what we can do is
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say I would like to find me the ratings
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that have let's say prime numbers as ratings,
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it's kind of silly, but whatever,
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here we go, so those are the prime numbers between one and ten,
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and we could say I would like to find all the ratings where the value,
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one of the values right, remember they have multiple ratings
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but one of the values is actually in this set,
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so the way this usually manifests is like go to the database
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and maybe I pull back some items, and it's got like a sub array of let's say ids
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and then I can go back to the database
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and say give me all the items in this other collection
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where the idea is in one of this like sub ids,
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so an example might be in the Talk Python Training stuff
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that remember the course contains all the chapter ids
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and I can go back into one single query
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that will give me all the chapters for a course it's this in operator, so let's try that.
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So there we go, apparently 69 thousand have a prime rating at some point
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not that that means anything, but it shows you how these operators work.
