Difference between revisions of "FIND Examples"

Revision as of 19:50, 21 June 2018

Introducing the `world` collection of countries

These examples introduce NoSQL using MongoDB. We will be using the find() command on the collection world:

By default, find() returns the entire contents of a collection. This is equivalent to find({})

Show all the documents in world

db.world.find();

find and findOne

It is also possible to just return the first document with findOne().

Show the first document of world

db.world.findOne();

skip and limit

It is also possible to specify which document to find by its position.

You can achieve the same with skip() and limit()

db.world.find().skip(49).limit(1)

Get the 50th document of world

db.world.find()[49];

Querying

find() can filter results if a document is used as an argument.

Get all the data concerning france

db.world.findOne({"name":"France"});

Using a projection with find

By passing a second parameter to find() the output can be limited to certain field(s)
In this example 1 indicates "true" and 0 indicates "false"

A feature of MongoDB is the ObjectID or _id.
This is a unique ID MongoDB adds to each document. Unlike other keys, it has to be explicitly set to false to be excluded from the results set.

Get the population of Germany

db.world.findOne({"name": "Germany"},{"population": 1,"_id": 0});

Comparisons $eq $gt $gte $lte $ne

MongoDB also allows comparisons. Syntax:


Mongo | MySQL
--------------
$eq   | == 
$gt   | >
$gte  | >=
$lt   | <
$lte  | <=
$ne   | !=, <>
$in   | IN
$nin  | NOT IN

List the countries with a population that's less than 1 million.

db.world.find({"population": {"$lt": 1000000}}, {"name": 1,"_id": 0});

Logical operations $and $or $not

It's also possible to have multiple conditions for use in an $and, $or, etc. This can be done in several ways, for example:


AND (implicit)
db.<collection>.find({<first condition>,<second condition>});
db.world.find({"population":{"$lt":1000000},"area":{"$gt":200000}});

AND (explicit)
db.<collection>.find({"$and":[<first condition>,<second condition>]});
db.world.find({"$and":[{"population":{"$lt":1000000}},{"area":{"$gt":200000}}]});

OR
db.<collection>.find({"$or":[<first condition>,<second condition>]});
db.world.find({"$or":[{"population":{"$lt":1000000}},{"area":{"$gt":200000}}]});

Find the countries with less than 1 million people, but over 200000km² area.

db.world.find({"population": {"$lt": 1000000},"area": {"$gt": 200000}},{"name": 1,"_id": 0});

$in and $nin

Lists can be used with $in and $nin:

Find the continent of Brazil, the United Kingdom, and Ghana.

db.world.find(
   {name: {$in: ['Brazil', 'Ghana', 'United Kingdom']}},
   {name: 1, continent: 1, _id: 0}
);

Regular Expressions

Pattern matching is possible with Regular Expressions (RegEx)

Show each country that begins with G

Reference

db.world.find(
  {name: {$regex: /^G/}},
  {name: 1, continent: 1, _id: 0}
);

@@ Line 5: / Line 5: @@
 By default, <code>find()</code> returns the entire contents of a collection. This is equivalent to <code>find({})</code>
 <p class="strong">Show all the documents in world</p>
-<pre class="def"><nowiki>db.world.find();</nowiki><</pre>
+<pre class="def"><nowiki>db.world.find();</nowiki></pre>
 </div>