3.md

https://docs.mongodb.com/manual/reference/operator/query/

Querying With single Criteria

{ : { $eq: } } db.inventory.find( { qty: { $eq: 20 } } ) The query is equivalent to:

db.inventory.find( { qty: 20 } )

Querying With Multiple Criteria

We can query based on multiple criteria by passing in comma-separated queries.

db.potions.find( { "vendor": "Kettlecooked", "ratings.strength": 5 } )

Comparison Query Operators

We can use comparison query operators to match documents based on the comparison of a specified value. $gt greater than $gte $lt $lte Syntax: { field: { $lte: value} } Consider the following example which uses the $lt operator with a field from an embedded document:

db.inventory.update( { "carrier.fee": { $lte: 5 } }, { $set: { price: 9.99 } } ) This update() operation will set the price field value in the documents that contain the embedded document carrier whose fee field value is less than or equal to 5

$and https://docs.mongodb.com/manual/reference/operator/query/and/#op._S_and

Exists and Not Equal To

Consider the following example:

db.inventory.find( { qty: { $exists: true, $nin: [ 5, 15 ] } } ) This query will select all documents in the inventory collection where the qty field exists and its value does not equal 5 or 15.

$ne greater than or equal to less than less than or equal to not equal to Common

We can query with a range by combining comparison operators.

db.potions.find({"price": {"$gt":10, "$lt": 20}}

Be Careful When Querying Arrays With Ranges

db.potions.find( {"sizes" :{"$gt": 8, "$lt": 16} }

)

Each value in the array is checked individually. If at least 1 array value is true for each criteria,

the entire document matches.

Conversely, the document will not match if only 1 criteria is met.

$elemMatch

{ : { $elemMatch: { , , ... } } }

The $elemMatch operator matches documents that contain an array field with at least one element that matches all the specified query criteria

field is an array here

Introducing Projections

find() takes a second parameter called a “projection” that we can use to specify the exact fields we want back by setting their value to true.

Only retrieve what’s needed

db.potions.find( {"grade": {"$gte": 80}}, {"vendor": true, "name": true} )

{ "_id": ObjectId(...), "vendor": "Kettlecooked", "name": "Shrinking" }

Excluding Fields

{"vendor": false, "price": false}

Excluding the _id

{"vendor": true, "price": true, "_id": false }

Either Select or Exclude Fields

Whenever projecting, we either select or exclude the fields we want — we don’t do both.

We can use the sort() cursor method to sort documents.

"price is the field to sort db.potions.find().sort({"price": 1})

"price": 1 , 1 means in descending order

Basic Pagination

We can implement basic pagination by limiting and skipping over documents. To do this, we’ll use the skip() and limit() cursor methods.

db.potions.find().limit(3)

We can implement basic pagination by limiting and skipping over documents.

db.potions.find().skip(3).limit(3)