MongoDB (6): Querying documents

Find () method

To query data from a MongoDB collection, you need to use MongoDB'sfind()method.

Grammar

find()The basic syntax of the command is as follows:

>db.COLLECTION_NAME.find(document)

Shell

find()Method will display all documents in an unstructured manner.

Pretty () method

To display the results in a formatted manner, you can use thepretty()method.

Grammar

> db.mycol.find().pretty()

Example

>db.mycol.find().pretty()
{
   "_id": 100,
   "title": "MongoDB Overview", 
   "description": "MongoDB is no sql database",
   "by": "yiibai tutorials",
   "url": "http://www.yiibai.com",
   "tags": ["mongodb", "database", "NoSQL"],
   "likes": "100"
}
>

In additionfind()to the method, there is afindOne()method that returns only one document.

the equivalent Where clause for MongoDB and RDBMS

To query a document based on some criteria, you can use the following actions.

Operation	Grammar	Example	RDBMS equivalent Statement
Equal	{<key>:<value>}	db.mycol.find({"by":"yiibai"}).pretty()	where by = ‘yiibai‘
Less than	{<key>:{$lt:<value>}}	db.mycol.find({"likes":{$lt:50}}).pretty()	where likes < 50
Less than or equal	{<key>:{$lte:<value>}}	db.mycol.find({"likes":{$lte:50}}).pretty()	where likes <= 50
Greater than	{<key>:{$gt:<value>}}	db.mycol.find({"likes":{$gt:50}}).pretty()	where likes > 50
Greater than or equal	{<key>:{$gte:<value>}}	db.mycol.find({"likes":{$gte:50}}).pretty()	where likes >= 50
Not equal to	{<key>:{$ne:<value>}}	db.mycol.find({"likes":{$ne:50}}).pretty()	where likes != 50

Here we will show you all the actions in the table above-

and operator in MongoDB

Grammar

In afind()method, MongoDB treats it as a condition if it is passed by using '，separate ' multiple keysAND. The following isANDthe basic syntax-

>db.mycol.find(
   {
      $and: [
         {key1: value1}, {key2:value2}
      ]
   }
).pretty()

Example

The following example shows all theyiibai tutorialstutorials written by "" and titled "MongoDB Overview."

> db.mycol.find({$and:[{"by":"yiibai tutorials"},{"title": "MongoDB Overview"}]}).pretty()
{
        "_id" : 100,
        "title" : "MongoDB Overview",
        "description" : "MongoDB is no sql database",
        "by" : "yiibai tutorials",
        "url" : "http://www.yiibai.com",
        "tags" : [
                "mongodb",
                "database",
                "NoSQL"
        ],
        "likes" : 100
}
>

For the example given above, the equivalent SQLwhereclause is-

SELECT * FROM mycol where by =‘yiibai tutorials‘ AND title =‘MongoDB Overview‘

Sql

You canfindpass any number of key values in the clause.

the OR operator in MongoDB

Grammar

You need to use keywords when you want to query a document based on aORcondition$or. The following is theORbasic syntax for a condition-

>db.mycol.find(
   {
      $or: [
         {key1: value1}, {key2:value2}
      ]
   }
).pretty()

Example

The following example displaysyiibai tutorialsall tutorials written or titled "MongoDB Overview".

>db.mycol.find({$or:[{"by":"yiibai tutorials"},{"title": "MongoDB Overview"}]}).pretty()
{
   "_id": 100,
   "title": "MongoDB Overview", 
   "description": "MongoDB is no sql database",
   "by": "yiibai tutorials",
   "url": "http://www.yiibai.com",
   "tags": ["mongodb", "database", "NoSQL"],
   "likes": "100"
}
>

using and and or conditions together

Example

The following example displayslikes10all documents that are greater than and are titled "MongoDB Overviewor"yiibai tutorials. The equivalent SQL WHERE clause is-

SELECT * FROM mycol where likes> 10 AND(by =‘yiibai tutorials‘ OR title =‘MongoDB Overview‘)

Sql

>db.mycol.find({"likes": {$gt:10}, $or: [{"by": "yiibai tutorials"},
   {"title": "MongoDB Overview"}]}).pretty()
{
   "_id": 100,
   "title": "MongoDB Overview", 
   "description": "MongoDB is no sql database",
   "by": "yiibai tutorials",
   "url": "http://www.yiibai.com",
   "tags": ["mongodb", "database", "NoSQL"],
   "likes": "100"
}
>

querying embedded/nested documents

Here's how to usedb.collection.find()the: method for an example of a query operation on an embedded/nested document. The samples on this page use theinventorycollection. To populate the inventory (inventory) collection to prepare some data, run the following command:

db.inventory.insertMany( [
   { item: "journal", qty: 25, size: { h: 14, w: 21, uom: "cm" }, status: "A" },
   { item: "notebook", qty: 50, size: { h: 8.5, w: 11, uom: "in" }, status: "A" },
   { item: "paper", qty: 100, size: { h: 8.5, w: 11, uom: "in" }, status: "D" },
   { item: "planner", qty: 75, size: { h: 22.85, w: 30, uom: "cm" }, status: "D" },
   { item: "postcard", qty: 45, size: { h: 10, w: 15.25, uom: "cm" }, status: "A" }
]);

Matching embedded/nested documents

To specify equality criteria on a field that is an embedded/nested document, use the query filter document{<field>：<value>}, which<value>is the document that you want to match.

For example, the following querysizeselects{ h: 14, w: 21, uom: "cm" }All documents that have a field equal to:

db.inventory.find( { size: { h: 14, w: 21, uom: "cm" } } )

An equality match in an entire embedded document requires an exact match to the specified<value>document, including the order of the fields.
For example, the following queryinventorydoes not match any of the documents in the Inventory () collection:

db.inventory.find(  { size: { w: 21, h: 14, uom: "cm" } }  )

Querying nested fields

To specify a query condition on a field in an embedded/nested document, use the dot symbol ("field.nestedField").

Specify equals match on nested fields

The following example selectssizeall documents that are nested in fields equal to "uomin":

db.inventory.find( { "size.uom": "in" } )

Use the query operator to specify a match

Query filter documents can be specified by using the query operator, such as the following form of conditions:

{ <field1>: { <operator1>: <value1> }, ... }

The following query uses thesizehless-than operator () in fields that are embedded in fields$lt:

db.inventory.find( { "size.h": { $lt: 15 } } )

Specifying and Conditions

The following query selectshAll documents that have a nested field that is less than15, and the nested fielduomequals "in" and thestatusfield equals "D":

db.inventory.find( { "size.h": { $lt: 15 }, "size.uom": "in", status: "D" } )

MongoDB (6): Querying documents